Phase Equilibria in the System GeO₂‐Al₂O₃

Abstract

The low-temperature a-modification is formed by a distortion of the hexagonal @-NbC. The distorted modification may be thought of as being formed by compressing the hexagonal parent cell perpendicular to one of the principal prism planes, thus effecting an approximately equal (but opposite in sign) length change of the base plane vectors. This behavior is immediately reflected by comparing the hexagonal pseudoaxes of alH = 3.092 A and = 3.145 A in the orthorhombic structure with the base length of U H = 3.118 A in the hexagonal subcell of @-NbC. The dimensions of the orthorhombic type are related to the above pseudoaxes by ao = 4 a l~. bo = 2aw 4 3 , Co = CH For a-V?C, the corresponding dimensions are alN = 2.872 A, and aZH = 2.904 A, compared to a H = 2.884 A for the hexagonal high-temperature modification. The distortion of the hexagonal modification in vanadium carbide (1.17,) is somewhat less than in a-NbC (1.7%).A problem not yet fully understood concerns the effect of composition on the order-disorder reaction as well as the type of order itself. 'While there is little doubt that the highertemperature transformations in Ta2C and N b C proceed as homogeneous reactions, and reversibility of the low-temperature transitions also appears established, the question whether the latter transitions proceed as homogeneous or heterogeneous reactions is not yet completely resolved.Present indications, however, favor a two-phased process since intermediate states, showing the coexistence of the a-and &modification, could be produced by insufficient heat treatments of the alloy specimens. Supporting this interpretation is the observation that the reaction generally proceeded much more rapidly in dense than in porous alloys, suggesting a nucleation and growth mechanism.Other observations,14 such as the much higher reaction rates obtained in excess metal compared to excess monocarbidecontaining alloys, are probably related to epitaxial dissolution-and-growth phenomena associated with the pronounced oriented metal precipitations in the subcarbide which could promote the formation of the new phase. Similarly, epitaxial growth phenomena varying with the stoichiometry of the phases probably are responsible for the observed slight differences in the structures formed in the transformation processes.

Reduction of Interface States Density Due to Post Oxidation with Formation of AlGeO Layer at Al2O3/Ge Interface

“…It is found that, from this value, the composition ratio of the Al oxide and the Ge 3+ component is 3 to 1. This ratio corresponds well to the ratio of a stable AlGeO phase, which is Al 6 Ge 2 O 13 (16). In addition, we also found that the crystallized AlGeO layer was formed at the Al 2 O 3 /Ge interface with PTO at 550°C for 30 s through observations of cross-sectional transmission electron microscopy and transmission electron diffraction (TED) (11).…”

Crystallization of GeO2–Al2O3–Bi2O3 Glass

“…As was reported in [42] the mullite-like phases, such as the germanium-mullite types Al2GeO5, Al2,26Ge0,74O4,87, Al2,38Ge0,62O4,81, and Al2,4Ge0,6O4,8, tended to crystallize rapidly. These crystalline phases were dispersed in the amorphous phases of glass.…”

The system Ga2O3(Al2O3)–GeO2(SiO2) studied by NMR, XRD, IR and DTA

“…Schneider and Werner 7 studied the high temperature reactions of GeO 2 , Al 2 O 3 and Ga 2 O 3 , ®nding that andalusite-type germanates were favored at lower temperatures and Al contents, while higher temperatures and Al contents favored 3 : 2 mullite-type germanates. Phase equilibrium studies in the GeO 2 ±Al 2 O 3 system predict only germanium±mullite (3Al 2 O 3 ?2GeO 2 or Al 6 Ge 2 O 13 ), 8 but two other Al±Ge±O phases are also known. These are Al 2 GeO 5 , which has the kyanite structure 9 and the digermanate Al 2 Ge 2 O 7 , which, unlike thortveitite (Sc 2 Ge 2 O 7 ), 10 contains AlO 5 bipyramids.…”