Chu Kun Kuo

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.

Epitaxial growth of Naβ″-Al2O3 single crystal film on α-Al2O3

Abstract Epitaxial Naβ″-Al 2 O 3 single crystal films have been grown into sapphire (0001)-substrates. The kinetic measurements and calculation indicate that the β″-Al 2 O 3 crystal is formed via chemical reactions between the gaseous alkali-species and sapphire substrate which are controlled by diffusion of oxygen species in the solid phase. The mechanism and crystallographic translations of film-growth are discussed.

The ionicity of binary oxides and silicates

The ionicity of binary oxides is presented as a function of the inverse of cation radius. The molar refractivity of silicates calculated on the basis of the ionicity of component oxides agrees with measured values. Discrepancies were found in the Pb and Fe silicates and these are explained by the metallic bonding portion in the basic oxide components.

The ion-exchangeability of β″-Al2O3 single crystal films

Abstract The ion exchange of epitaxially-grown β″-Al 2 O 3 single-crystal films has been investigated. The X-ray crystallographic parameters and optical-refractive-index data for the isomorphic Na-, Li-, K- and Ag-β″-Al 2 O 3 films have been determined.

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.

Stereography of growth of β″-Al2O3 single crystal films from sapphire substrates

Abstract A model is presented that simulates the growth of Na-β″-Al 2 O 3 single-crystal films from sapphire substrates in alkali atmospheres. The formation of β″-Al 2 O 3 occurs via NaO insertion, α-Al 2 O 3 oxygen lattice transition and Al 3+ ion redistribution. Calculations are presented, based on the chemical and structural free energy changes and the stresses induced by the necessary structural mismatch. Oxygen diffusion is examined at 1235–1350°C on the basis of the model proposed and the microstructure development explained.

Phase equilibrium calculations in the systems (Na, Li)-ß′-Al2O3O2 and (Na, Li)-ß′-Al2O3O2H2O in relation to Na-ß′-Al2O3 single-crystal-film growth

Abstract Equilibrium and metastable-equilibrium phase relationships and gas-species distributions in the systems: (Na 1− x Li x )-s′-Al 2 O 3 O 2 ( x =0.05, 0.1, 0.2, 0.3 and 0.5) and (Na 1− x Li x )-s′-Al 2 O 3 O 2 H 2 O ( x =0.05) at 1400–1800 K have been calculated. The partial pressure, composition of solids and distribution in the gas phase are given, and the effects of oxygen and minor water discussed. The consequence of these results on the formation of large-area, single-crystal Na-s′-Al 2 O 3 films by solid-vapour reaction is identified.

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.

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 refractometry of β- and β″-Al2O3-type aluminates and gallates

Abstract The ionicity of oxide and β- and β″-Al 2 O 3 crystals is identified via Lorentz–Lorenz molar refractivity. The investigation was extended to sodium gallates, the refractivity and average refractive index of which are estimated.