On some aspects of breakdown of β″-alumina solid electrolyte

A theoretical expression is derived for the pressure generated in the sodium-filled cracks of -alumina under electrolytic conditions by treating the flux of sodium ions to the cracks in terms of the Laplace equation for the appropriate boundary conditions. It is demonstrated that the pressure generated decreases with increasing crack length for a given current density in contrast to the predictions of some investigators. It is suggested that some other factors must be considered if the microfracture model via Poiseuille pressure is to be a viable mechanism for electrolyte degradation.     

Zirconia-toughened sodium beta″-alumina solid electrolyte

The effect of a 15 vol% zirconia dispersion on the critical current density for failure initiation of beta-alumina solid electrolyte was examined. Single phase and composite electrolytes were tested in standard sodium-sodium test cells and subjected to increasing ionic currents. Onset of degradation in the electrolyte was detected by monitoring acoustic emissions from the cell. Preliminary examination of the electrolyte material showed that the problem of producing a uniform dispersion of zirconia in pure beta-alumina had not been solved. However, the electrolytes were of sufficient quality to draw important conclusions about the potential of transformation toughening for improving electrolyte performance.     

Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

Structure,properties and production of β-alumina

The crystal structures of the -alumina compositions have been described and used to explain the fast ion transport for which these materials are renowned. Measured values of both the single crystal and polycrystalline ionic conductivity show a wide variation; this is explained in terms of the range of chemical compositions of the -alumina system and also the variety of measuring techniques used. Dopants or impurity ions can have a significant effect on the physical properties of the -aluminas. The ionic conductivity, the stability of the material and the densification during sintering have been considered in relation to the nature and level of a range of dopants described in the literature. The optimization of the ionic and mechanical properties has been achieved by development of the fabrication techniques and it is this which accounts for much of the present research. Thus the many different methods of producing both single and polycrystalline material have been described, including the range of sintering routes currently available. The advantages and disadvantages of each production route in terms of the resulting properties have also been discussed.     

Crystal growth of thin-plate β-alumina by liquid transport

Single crystals of -alumina (Na₂0.11Al₂O₃) were grown by liquid transportat 1030–1150 °C and for durations of 4–140 h, using an Na₃AlF₆ solvent and - or -alumina as the nutrient. The maximum size of -alumina crystal grown using a small platinum crucible was 1.1 cm in length and 0.6 mm in thickness. On the other hand, a single crystal 1.8 m long and 0.7 mm thick was grown using a large crucible. These grown single crystals were hexagonal, thin platelike, and very transparent. Screw dislocation-like patterns were observed on the surfaces of grown -alumina single crystals; each pattern showed a right-handed rotation. Small crystals on a matrix crystal grew spirally about the c-axis and formed a new, flat surface on the matrix crystal, evidently an intrusive twin. The angle between the two inclined crystals was 2.9–2.8 °. The twin face seemed to be w(14, 14, 3). The growth rates of -alumina single crystals treated for less than 12 h using large and small crucibles were 3.3 × 10^–1 and 4.0 × 10^–1 mm h^–1, respectively. For treatment ranging from 12 to 40 h growth rates were 1.1 × 10^–1 and 1.5 × 10^–1 mm h^–1 for the large and small crucible, respectively; rates for 40–100 h treatment were 0.3 × 10^–1 mm h ^–1 for both crucibles. The grown -alumina single crystals had fairly good crystallinity. The growth mechanism of the -alumina single crystal can be estimated from transport through liquid film in the crucible, i.e. a creeping phenomenon.     

Wetting characteristics of sodium on β″-alumina and on nasicon

The sessile drop technique was used to determine the wetting characteristics of liquid sodium on -alumina and nasicon (Na₃Zr₂Si₂PO₁₂). The effects of moisture and temperature were studied using the wetting experiments. A range of contact angles (from 0 to 160°) were observed. The samples exposed to moisture exhibited poor wetting. The fact that good wetting is a prerequisite for improved cell performance was demonstrated via Na ¦-alumina¦ Na cell tests. Cells baked out in a vacuum passed in excess of 1100 Ah cm^–2 at 4 A cm^–2 current density without any degradation. Unbaked cells under similar conditions failed under 200 Ah cm^–2.     

Microstructural and chemical stability of Y-ZrO2 reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of -alumina reinforced with 10 vol% of tetragonal partially stabilized 3 mol% Y₂O₃-ZrO₂ (3Y-ZrO₂) and with 10 vol% of cubic 8 mol% Y₂O₃-ZrO₂ (8Y-ZrO₂) in molten sulfur or molten Na₂S₄ has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 °C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into -alumina and immersed in molten Na₂S₄. Destabilization without incorporation into -alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of -alumina enhanced zirconia destabilization in that -alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y₂O₃ from partially stabilized 3Y-ZrO₂. From XRD analyses, changing from partially stabilized 3Y-ZrO₂ to cubic 8Y-ZrO₂ in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO₂ suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y₂O₃ and the corrodants. In order to avoid depletion destabilization of zirconia in -alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO₂ is used.     

Synthesis of γ-alumina via precipitation in ethanol

Nanosized γ-alumina powder was prepared via sol-gel precipitation in ethanol followed by a washing-drying treatment and calcination. The reactant concentration, the molar ratio of reactants and the calcination temperature were inspected by homogenous design. BET, TEM, XRD and IR were used to characterize the products. The experimental results showed that ethanol can avoid hard agglomerate during precipitation and that calcination temperature was the major factor to determine the surface area, the diameter and the phase composition of the final product. When the calcination temperatures were controlled between 686-1029 °C, the agglomerate-free nano-sized γ-alumina particles with a size range of 5-9 nm were obtained.     

Role of Edge and Screw Dislocations in Deformation of Metals

It was widely accepted that the screw dislocation is responsible for the strong temperature dependence of the yield stresses observed in bcc metals. In this paper, we show the role of edge dislocations in the deformation of metals and point out that in some cases, its main contribution to the plastic flow behaviour cannot be ignored.     

Theory of Nucleation on Dislocations

Cahn's theory of nucleation on dislocationshas been analyzed thoroughly and the imperfectionof the theory in some cases has been discussed.Arevised theory has been proposed with applicationto the problem of precipitation of M(CN)phase inaustenite.     

Preparation of β-FeSi2 thermoelectric material by laser sintering

Laser sintering has been applied for preparing β-FeSi₂ based thermoelectric alloy for the first time. Effects of laser sintering time on alloying, phase transformation and microstructure of FeSi₂ were investigated by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Effects of annealing temperature and time on phase transformation were also studied through Seebeck coefficients. The results show that for 90 s laser-sintered samples, it takes only 15 h to obtain β phase under Ar atmosphere followed by an annealing at 1073 K. These samples exhibit homogeneous microstructure with average grain size of less than 5 μm. A maximum Seebeck coefficient at room temperature could reach 115 μV/K. It indicates that laser sintering could be an alternate faster preparation method to generate high quality β-FeSi₂ thermoelectric material with little contamination due to its advantages of rapid heating rate, high cooling rate and rapid solidification.     

Mechanochemical synthesis and properties of thermoelectric material β-FeSi2

The mechano-chemical synthesis of thermoelectric material on the basis of -FeSi₂ has been investigated. The mixture of FeSi and amorphous Si has been shown to be a optimum precursor to produce the thermoelectric ceramics. The ceramics properties (thermoelectric power , V/K, electrical conductivity 1/*cm) have been considerably improved by means of doping with superequilibrium quantity of 12% of aluminium (substitution of silicon) or 10% of cobalt (substitution of iron). The mechanical alloying in a high energy ball mill, under the acceleration of treating balls 800 m/sec² produced homogeneous powder with a superequilibrium quantity of dopant, which conversed into thermoelectric ceramics after short annealing in vacuum at low temperature (780°C). The samples of ceramics with the maximum content of doping elements have increased thermoelectromotive force-up to 800 V/K. Mechanically alloyed ceramic is a promising material as a medium temperature thermoelectric with advanced properties for autonomous power supply units.     

Formation of γ-alumina nanorods in presence of alanine

Boehmite and alumina nanostructures were prepared using a simple green sol-gel process in the presence of alanine in water medium at room temperature. The uncalcined (dried at 200 °C) and the calcined materials (at 500, 600 and 700 °C for 4 h) were characterized using XRD, TEM, SEM, N₂ physisorption and TGA. Nanorod aluminas with a possible hexagonal symmetry, high surface area and relatively narrow pore size distribution were obtained. The surface area was enhanced and crystallization was retarded as the alanine content increased. The morphologies of the nanoparticles and nanorods were revealed by a transmission electron microscope (TEM).     

Dislocations in the Crystallites of Nanocrystalline α-Fe —A Molecular Dynamics Study

Molecular dynamics simulations are carried out in order to Study the atomic structure of crystalline component of nanocrystalline α-Fe when it is consolidated from small grains. A two-dimensional computational block is used to simulate the consolidation process. All the preset dislocations in the original grains glide out of them in the consolidation process, but new dislocations can generate when the grain size is large enough. It shows that dislocations exist in the consolidated material rather than in the original grains. Whether dislocations exist in the crystalline component of the resultant model nano-material depends upon grain size. The critical value of grain size for dislocation generation appears to be about 9 nm. This result agrees with experiments qualitatively.     

Hydrogen Trapping by Dislocations and Grain Boundaries in Fe-3%Si AHoy

In present work,Fe-3%Si alloy specimens with various degrees of cold working and various grainsizes were chosen to determine the effect of cold working and grain sizes on the hydrogendiffusivities by electrochemical permeation method and the mechanism of hydrogen trapping by dis-ocations and grain boundaries is discussed.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

Atomistic Simulation of Interaction between Grain Boundary and Dislocations in Ni_3Al

The embedded atom type potentials and static relaxation method combined with a steepest decentcomputational technique have been used to simulate the interaction between the grain boundary(GB) and dislocations in Ni_3Al alloys.The focus has been placed on the energy feature of theinteraction,the distortion of GB structural units,and the dislocation core structure near the GB.Im-plication has also been made on the results for the understanding of the mechanism responsible forB-enhanced ductility.