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.     

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.     

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.     

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.     

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.     

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.     

Effect of hydrolysed aluminium treatment on rheological characteristics of α-alumina slurries

This paper describes the process for the hydrolysed aluminium treatment (HA) on alumina surface and its influence on the rheological characteristics of alumina slurries. Three different commercial grade aluminas were provided with the surface treatment with hydrolysed aluminium using aluminium nitrate and hexamethylenetetramine (HMTA) under controlled conditions. The HA treatment increased the concentration of Al-OH surface sites resulting in higher H⁺ adsorption on the alumina surface. A highly concentrated (>55 vol%) electrostatically stabilized alumina slurry was prepared from HA treated alumina powders. The rheology of such slurry was studied and the results on the viscosity and yield stress are presented. The alumina slurries followed the Casson Model flow behaviour. The wet and flow behaviour of the alumina with and without HA treatment was also studied and the results are compared. The surface treatment showed the advantage of maintaining low viscosity and yield stress of alumina slurries even at higher solids loading (>55 vol%) that are prepared in the acidic aqueous medium. The results on viscosity and yield stress were compared with that of the polyelectrolyte dispersed system.     

Colloidal nanometric particles of nickel deposited on γ-alumina: characteristics and catalytic properties

The present paper deals with supported nanoparticles of metallic nickel prepared by impregnation of an organometallic colloidal precursor (NiRC complex) on -alumina. The obtained supported materials (NiRCS complex) were characterized by STEM, EELS, XRD and TGA and tested in the hydrogenation reaction of benzene and styrene both in the gas and liquid phases. EELS and STEM studies showed that the metallic nickel nanoparticles of the NiRC precursor were well dispersed (1–3 nm) on the support, without size variation during the impregnation step. However, particle growth in NiRCS was observed in samples used after reaction tests. On the other hand, TGA experiments showed that the nickel phase was inserted in an organic matrix which remained in the supported material after impregnation. This organic matrix should be partly removed or totally decomposed by washing with an appropriate solvent or thermal treatment above about 250°C under N₂, H₂ or air atmosphere respectively. The supported nickel particles were active in the gas phase hydrogenation of benzene in the temperature range of 150–200°C and their activity was enhanced by partial removal of the organic matrix with an organic solvent (e.g. ethanol) or by its total thermal decomposition. They were also found very active in the gas phase hydrogenation of styrene at room temperature. Finally, the supported nickel particles exhibited good hydrogen reservoir properties in organic solvent media.     

The relationship between the a.c. impedance and microstructure of a sodium β-alumina ceramic

A comparison is made between the microstructures, resistivities and strengths of sodium /-alumina samples prepared under different firing conditions. Double peak firing schedules are shown to result in samples of higher strength with a narrower grain-size distribution and lower resistivity than those produced by single peak firing. Complex plane a.c. impedance and admittance data show the sodium ion conduction over the temperature range 173 to 300 K to be governed primarily by intergranular regions of the sample (with the resistance varying with grain size) whilst resistivity measurements at 623 K show the conduction to be primarily intragranular controlled at elevated temperatures.     

A supersaturation model for the degradation of sodium β/β″-aluminas

A mechanism is proposed for the sodium degradation of / alumina electrolytes. It is based on the Na⁺ supersaturation of feeder grains peripheral to flaws in the liquid-sodium/ electrolyte interface. This supersaturation is brought about by the focusing of the current by the flaw and leads to local oxidation of oxygen ions in the aluminate structure. This process leads to the formation of colour-centres and sodium atoms in the grains. The latter coalesce to form sodium colloids, microcracking the microstructures. These microcracks join the originating flaw and promote its extension. A model is presented which demonstrates the feasibility of the proposed degradation mechanism and it is discussed in light of other associated physical phenomena.     

Effect of the starting Al2O3 and of the method of preparation on the characteristics of Li-stabilized β″-Al2O3 ceramics

The influence of the morphology and the size of the particles of various types of starting Al₂ O₃ material on the synthesis and characteristics of Li-stabilized'-Al₂O₃ ceramics have been investigated. The use of highly dispersed oxides makes it possible to attain higher densities in the fired ceramic bodies due to their higher reactivity. In the case of oxides obtained from ammonium alum, the degree of dispersion and the reactivity may be increased by raising the amount of-Al₂O₃ up to a certain limit. Alumina prepared from Al₂ (OH)₅ NO₃ by slurry solution spray-drying also gives satisfactory results despite its lower degree of dispersion. This is connected with the morphology of the particles. In the case of synthesized materials containing an insufficient amount ofAl₂O₃-NaAlO₂ eutectic, high densities may also be achieved by applying a two-step firing schedule at temperatures above the melting point of the eutectic.     

Formulation studies on ibuprofen sodium–cationic dextran conjugate: effect on tableting and dissolution characteristics of ibuprofen

The effect of electrostatic interaction between ibuprofen sodium (IbS) and cationic diethylaminoethyl dextran (Ddex), on the tableting properties and ibuprofen release from the conjugate tablet was investigated. Ibuprofen exhibits poor flow, compaction (tableting) and dissolution behavior due to its hydrophobic structure, high cohesive, adhesive and viscoelastic properties therefore it was granulated with cationic Ddex to improve its compression and dissolution characteristics. Electrostatic interaction and hydrogen bonding between IbS and Ddex was confirmed with FT-IR and DSC results showed a stepwise endothermic solid–solid structural transformation from racemic to anhydrous forms between 120 and 175?°C which melted into liquid form at 208.15?°C. The broad and diffused DSC peaks of the conjugate granules as well as the disappearance of ibuprofen melting peak provided evidence for their highly amorphous state. It was evident that Ddex improved the flowability and densification of the granules and increased the mechanical and tensile strengths of the resulting tablets as the tensile strength increased from 0.67?±?0.0172 to 1.90?±?0.0038?MPa with increasing Ddex concentration. Both tapping and compression processes showed that the most prominent mechanism of densification were particle slippage, rearrangement and plastic deformation while fragmentation was minimized. Ddex retarded the extent of dissolution in general, indicating potentials for controlled release formulations. Multiple release mechanisms including diffusion; anomalous transport and super case II transport were noted. It was concluded that interaction between ibuprofen sodium and Ddex produced a novel formulation with improved flowability, tableting and dissolution characteristics with potential controlled drug release characteristics dictated by Ddex concentration.     

Direct synthesis and characterization of the Fe3+ analogue of Naβ″-alumina

A new isomorphic analogue of-alumina, a potassium-free Na-ferrite (Na₂O · 5 (Al_0.10Fe_1.90)O₃, referred to as Na-ferrite), has been synthesized and is characterized using X-ray powder and single crystal diffraction, wet chemical analysis, and transmission electron microscopy (TEM). In addition, Na-aluminate-gallate-ferrate phases were also successfully synthesized, but were not characterized in detail. Na-ferrite grew in a slowly cooling melt or partial melt from 1250° C in air using Na₂CO₃ and Fe₃O₄ (magnetite) as starting materials. The compound was also synthesized using-Fe₂O₃ instead of Fe₃O₄ under the same conditions, but its abundance was much less and-Fe₂O₃ and NaFeO₂ were the dominant phases. The new material (cell dimensionsa = 0.5955,c = 3.5623 nm) is reversibly hygroscopic, andc increases 2% after hydration. TEM images dearly show the 1.19 nm lattice spacing corresponding to the ionic conduction planes, and these planes are observed to be both straight and curled. The origin of the curled lattice planes, the existence of which is also evident in the single crystal precession photographs, is not clear, although we believe it may involve a structural misfit between the ferric oxide spineI block and soda layer.     

Wetting and interfacial behavior of Ni–Si alloy on different substrates

Wetting of molten Ni–56 at.% Si alloy on different substrates (SiC ceramic, Ni- and Co-based superalloys, Kovar, and Mo) are performed under different experimental conditions by the sessile drop technique. Temperature, atmosphere, and substrate composition play the key roles in determining the wettability, the spreading characteristics, and the interfacial morphology of the final interfaces. The non-reactive wetting characteristics in Ni–Si/SiC system are confirmed, with a spreading rate increasing with temperature increasing. In the Ni–Si/metal systems the spreading process is determined by the competition between spreading along the substrate surface and the interfacial interactions. Excellent wettability and fast spreading are found in the Ni–Si/Co-based superalloy, Ni–Si/Kovar, and Ni–Si/Mo systems at both the temperatures (1100 and 1200 °C). These results can be used as a reference guide for joining SiC to these metallic components, or to itself, using the Ni–Si alloy as filler metal.     

Plane section and spatial characteristics of equiaxed β-brass grains

Abstract

A study was undertaken to define better the concept of the statistical grain. A cube having edges of 52·1 mm was cut from a slowly cooled β-brass ingot which had equiaxed grains. The distributions of sizes and shapes of grain sections on the faces of the cube were determined. The grains were then separated in a solution of mercurous nitrate and nitric acid. Observations were made of the relationship between grain diameter and the number and shapes of faces. For the 941 separated whole grains, the frequency of distribution of grain diameters obeyed the log–normal law. A plane distribution curve was derived for the statistical grain which enables spatial distributions to be calculated from measurements made on a plane section.

MST/825     

Phase transformation and structural characteristics of zinc-incorporated β-tricalcium phosphate

Biological performance of bioceramics such as calcium phosphate has been proved to be improved by substitution of different ions like Mg, Sr and Si. In this study, different amounts of Zn ions in nitrate form were incorporated into β-tricalcium phosphate in which various molar ratios of Ca:Zn were achieved: 3:0, 2.8:0.2, 2.6:0.4, 2.4:0.6, and 2.2:0.8. The mixtures were heated at different temperatures ranging from 800–1100 °C. The phase composition, amount of each phase and lattice parameters of β-tricalcium phosphate were determined by means of X-ray diffractometry and coupled software. Also, solubility of the heated mixtures was investigated by determining the amount of Ca and Zn released into a simulated body fluid during 120 h. The results revealed that only limited amount of Zn ions could be incorporated into β-tricalcium phosphate lattice and ZnO phase was formed when high content of zinc nitrate was introduced in initial mixture. Both a and c lattice parameters of β-tricalcium phosphate were reduced by adding Zn. The release rate of calcium ions into the simulated body fluid was approximately constant during 120 h while for Zn minor release was observed.     

Diffraction studies of β″-magnesium sialon

The compositions of high magnesium content -magnesium sialon crystals within the 3M/4X plane of the Mg-Si-Al-O-N system indicate some form of metal atom ordering wihtin the structure. Although it is not possible using X-ray diffraction to detect weak additional reflections arising from the ordering between magnesium, aluminium, and silicon atoms, such weak reflections are revealed on electron diffraction photographs for a phase, resulting in a tripling of the hexagonal latticec-dimension. These show to have a structure very similar to rhombohedral willemite (Zn₂SiO₄), with the (Mg, Al) metal atoms ordered with respect to the silicon atoms in a 21 ratio. However, there are some additional weak diffraction spots with indices not obeying the rhombohedral condition of –h+k+l=3n. It is proposed that the structure of this is identical to that of willemite and the extra spots are a result of some form of twinning, which implies the existence of ordered microdomains.     

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.     

Physical and thermal characteristics of aluminium titanate dispersed with β-spodumene and zirconia

The effects of -spodumene (Li₂O·Al₂O₃·4SiO₂) and zirconia additives on the physical and thermal characteristics aluminium titanate (AT) ceramics have been investigated. XRD, DTA, SEM, TEM and dilatometry have been used to characterise the influence of these additives on phase relations, densification, microstructure, thermal expansion, thermal decomposition and thermal shock behaviour of AT. The results show that the presence of both -spodumene and zirconia significantly reduces porosity, lowers thermal expansion, improves densification, hardness and thermal stability, without degradation of thermal shock resistance. Microstructural examinations have revealed the presence of glassy phase due to the phase separation of -spodumene and spontaneous microcracks due to thermal expansion anisotropy of AT.     

Comment on 'Diffusion of water and sodium counter-ions in nanopores of a β-lactoglobulin crystal: a molecular dynamics study'

Malek et al (2005 Nanotechnology?16 S522) studied the diffusion of water and sodium counter-ions in a β-lactoglobulin (βLG) crystal from a 5?ns molecular dynamics simulation. They presented water density in the main pore, and water mobility in the core and surface zones. Unfortunately, their results were incorrect and misleading. In this comment, we provide the results from our analysis for the same system.