Phases in the System NaAlO₂‐Al₂O₃

Abstract

Mixed oxide powders prepared by ashing filter paper soaked in aqueous salt solutions were heat‐treated in the range 800° to 1150°C. Compositions spanning the range NaAlO2 to pure Al2O3 were examined. A phase with the mullite structure, λ‐Na2O ·xAl2O3 with x varying from 3 to ≊ 12, and a new hydrated form of sodium aluminate, ρ‐Na2O · Al2O3· 6H2O, were found. The stability ranges and decomposition behavior of these materials were determined. A K analog to the A phase was also found.

Formation of ultradispersed systems based on sodium polyaluminates from thermal plasma flows

“…The decrease in the specific surface area of SPA is presumably related to the transformation of the orthorhombic structure m-Al 2 O 3 (similar to mullite, but with a doubled axis providing a more symmetric pseudotetragonal structure with cell parameters a = b = 0.768 nm, c = 0.291 nm) [6] into the rhombohedral structure of b²-aluminate with hexagonal cell parameters a = 0.561 nm and c = 3.346 nm (for the composition Na 1.72 Al 10.66 Li 0.30 O 17 ). In this case the phase transformation is accompanied by the SPA density increasing from 2.7 g/cm 3 for the m-phase to 3.28 g/cm 3 for b²-aluminate.…”

“…The mullite-like sodium aluminate is present in products of plasmachemical synthesis up to temperatures of 1100 -1150°C, although the limit of stable existence of m-Al 2 O 3 in solid-phase reactions does not exceed 1050 -1085°C [5,6].…”

Superstructure of Mullite-type KAl9O14

“…6 A new phase λ-Na 2 O· x Al 2 O 3 , exhibiting a wide compositional variation (3 ≤ x ≤ 12), was discovered by Elliot and Huggins. 7 This compound shows typical “mullite” lattice parameters a = 7.58, b = 7.68, and c = 2.88 Å. Mazza et al. 6 were able to index the powder diagrams of five boron-containing samples with a composition of Al 6– x B x O 9 (1 ≤ x ≤ 3) and two mullite-type alkali aluminates, reported as NaAl 6 O 9.5 and KAl 6 O 9.5 using a pseudotetragonal unit cell of a ≅ b ≅ 7.640 Å and c ≅ 2.937 Å and a ≅ b ≅ 7.708 Å and c ≅ 2.906 Å, respectively.…”

Sol–Gel Synthesis of λ–Na2O·xAl2O3 Films

“…A Na 2 O⅐xAl 2 O 3 (3 Ͻ x Ͻ 12) phase, known as -Al 2 O 3 , in which Na ϩ ions occupy interstitial positions in alternate closepacked layers in a mullite-like lattice, has frequently been observed by researchers preparing ␤-alumina powders. 5,6,11,12 The diffraction pattern of the -phase can be indexed using an orthorhombic cell as in the case of mullite, but the length of the two axes are nearly equal so that it may be treated as a pseudotetragonal cell. The -phase was reported to appear in the Na 2 O-Al 2 O 3 system by various preparation techniques, i.e., gel crystallization, 6 ashing filter paper, 12 thermal decomposition of a complex compound, 6 and double alkoxide method.…”

“…5,6,11,12 The diffraction pattern of the -phase can be indexed using an orthorhombic cell as in the case of mullite, but the length of the two axes are nearly equal so that it may be treated as a pseudotetragonal cell. The -phase was reported to appear in the Na 2 O-Al 2 O 3 system by various preparation techniques, i.e., gel crystallization, 6 ashing filter paper, 12 thermal decomposition of a complex compound, 6 and double alkoxide method. 2 In all methods cited above, peaks corresponding to the mullite-type phase were observed in powders fired at low temperatures, between 600°and 1050°C, before a complete transformation in pure ␤-alumina.…”