Aichun Tan

Formation and characterization of Naβ″-alumina single crystal films

Abstract Single crystal Naβ″-Al 2 O 3 films have been prepared by a vapour reaction technique. The formation of Naβ″ -Al 2 O 3 phase is initiated by introducing Li + into the furnace atmosphere. Laue back-reflection X-ray photographs and SEM observation show perfect Naβ″-Al 2 O 3 single crystal film grows in the (001) direction under properly controlled conditions. SEM indicates the thickness of the film ≥40 μm. The formation mechanism of the single crystal Naβ″-Al 2 O 3 film has been studied and the microstructure, conductivity and luminescence observed.

The development of stress during NaKion exchange of polycrystalline β, β″-Al2O3 and its influence on the ion-exchange process

Abstract Na + K + ion-echange at 810°C and 1200°C between high density β, β″-Al 2 O 3 multiphase specimens and KClNaCl melts has been investigated. It was found that the equilibrium conditions are affected significantly by the developed stresses. Microstresses are caused by the thermal expansion anisotropy of both phases and by the difference of alkali ion diffusion rates in the two phases. Macrostresses are introduced by compositional differences between the surface and interior of the specimens. The critical condition for the appearance of microfractures has been estimated using a simplified model. Based on the determination of the distribution gradient of the two kinds of ions, the residual macrostresses have also been calculated. Analysis of these two kinds of stress suggests failure of the material when conditions are such that they act concurrently.

Ion exchange and thermodynamic studies on (Na-K) β″-alumina

Abstract The solid-melt equilibria curve between Na β″-Al 2 O 3 powder and Na-K chloride melts at 810°C has been obtained together with an accurate determination of the β″-Al 2 O 3 cell parameters for the entire Na-K composition range. The activities and their coefficients, chemical potentials and free energies for the different Na-K compositions have been calculated. Analysis of these thermodynamic properties indicates that K + strongly prefers the β″-Al 2 O 3 phase and K β- and β″-Al 2 O 3 are thermodynamically more stable than the Na analogs. The Na + and K + ions repulsively interact for 0 + ] + ]>0.93. These interactions are reflected by increases and decreases of the a 0 lattice parameter.

The influence of grain-boundaries on the conductivity and ion-exchange rate of β″ -alumina polycrystalline isomorphs

Abstract The grain and grain-boundary electrical conductivities of polycrystalline β″ -aluminas were measured. It was found that grain-boundaries decrease the material conductivity substantially, especially when large ions are involved. The NH + 4 and H 3 O + ion-exchange rates are decreased significantly by grain-boundary effects. These observations are discussed in terms of grain-boundary structure and the degree of disorder thereat. Techniques for improvement of grain-boundary conductivity have been investigated.

Improved precursor ceramics for proton-conductive β''/β-aluminas

Optimum composition and preparation conditions have been determined for improved proton-conductive β″/β alumina precursors. The K+ content was decreased from 0.4 to 0.2 mole fraction, reducing the β-alumina phase fraction to <0.15. Extended-time (≤one hour) sintering techniques were developed which favor formation of β″-alumina and cleaner grain boundaries. The proton-conducting isomorphs of the new material (8N4) have higher conductivity than the originally-developed precursors (6N3), e.g., NH+4-8N4, 1.1×10−5 (Ω−1cm−1) versus NH+4-6N3, 7.3×10−6 (Ω−1cm−1) a t 25°C.

The influence of alkali content, stabilizer distribution and atmosphere on mixed-alkali ß″/ß-Al2O3 sinters

Abstract Studies were undertaken on the synthesis of (Na, K)-s″/s-Al 2 O 3 . It was found that the β- and s″-Al 2 O 3 phase transformations are strongly influenced by the total content of alkali ions, the Na + /K + ratio and the dissolution of Mg 2+ in the β- and s″-Al 2 O 3 phases. Microstructural development during the sintering process is discussed.

Preparation and characterization of textured polycrystalline Na and K-β″β-aluminas

Abstract Polycrystalline samples of Na, K-β″ β -aluminas with a c0-textured microstructure have been successfully produced. Very fine, high-aspect-ratio, freeze-dried powder was pressed into thin disks and sintered. The high degree of preferential orientation in the resultant ceramics was revealed by XRD and SEM examination. A significant difference of electric conductivity between directions perpendicular and parallel to the pressing direction was determined by ac impedance spectroscopy, i.e., 2.7×10−6Ω−1 cm−1 and 2.2×10−4Ω−1 cm−1 respectively at 25°C for K-β″ β -aluminas.

Structural determinations in mixed-alkali, mixed phase (β,β″)-aluminas

Abstract The structural determination of polycrystalline Na-K-β-and β″-aluminas with f ( β )=0.25∼0.35 by X-ray diffraction is described. It is shown that some characteristic peaks of the β-Al 2 O 3 and β″-Al 2 O 3 phases could be influenced by the possible existence of spinel (MgAl 2 O 4 ) in the MgO-stabilized polycrystals. The influence of the overlapping XRD peaks of the β- and β″-Al 2 O 3 phases on the determination of their lattice parameters is discussed.

The kinetics of ultrasonic/electrical-field assisted hydronium ion exchange of K+-β′′-Al2O3

Abstract The rate of electrical-field-assisted, hydronium-ion-exchange of K + -β″-Al 2 O 3 ceramics was investigated. A model based on ion migration from a decreasing source was proposed to interpret the kinetics of the exchange process. Results of the ion exchange experiment with ultrasonic field superimposition plus a cycling dc field are also reported.

Impedance analysis as a tool for designing β″-alumina microstructures

Abstract The ion conduction behavior of microstructurally designed β″-Al 2 O 3 ceramics was studied by ac impedance spectroscopy. The experimental results for textured alkali and ammonium β″-aluminas are presented. Impedance spectra characteristics and microstructures are related