Spinel NiAl Research Articles

Processing and Heat Treatment Effects on an Al2O3/Ni3Al Composite

Abstract Various processing routes may be used to fabricate Ni3Al matrix composites. In the present study, vacuum hot pressing and hot extrusion were used to fabricate an Al2O3 Composite particulate-reinforced Ni3Al composite. Relatively low extrusion temperatures promoted gamma-phase formation as did extended high temperature anneals. The room-temperature bend strength was found to increase as the gamma-phase content increased. Furthermore, the composite creep rate was affected by the. processing parameters and their effect on the amount of gamma/pgamma-prime interface. The matrix and particulates did not exhibit any reaction after normal processing. However, a minor reaction was noted after annealing near 1000°C, while extensive reaction resulting in Ni-Al spinel formation occurred above 1200°C in air

FT-113 study of the surface properties of the spinels NiAl 2O 4 and CoAl 2O 4 in relation to those of transitional aluminas

The surface chemistry of δ-Al 2O 3, NiAl 2O 4, and CoAl 2O 4 has been investigated comparatively by FT-IR spectroscopy using adsorbed carbon monoxide and pyridine as probe molecules. The surface OHs of the inverse spinel NiAl 2O 4 are very similar to those of δ-Al 2O 3 and other transitional aluminas while those of CoAl 2O 4 are different and resemble those of other normal spinels. Both carbon monoxide and pyridine adsorption provide evidence for the presence of tetrahedral Al 3+ ions exposed on the surface of the inverse spinel NiAl 2O 4. On the normal spinel CoAl 2O 4 octahedral Al 3+ ions predominate with respect to the tetrahedral ones, also present because of partial inversion. Ni 2+ and Co 2+ ions are also detected on the respective surfaces. Slight modifications to the literature criteria are suggested for the assignments of the IR bands to surface OHs and of adsorbed pyridine on aluminas.

The Early Stages of the Spinel-Alumina Phase Transformation

ABSTRACTThe initial stage of topotactic growth of Ni-Al spinel into Al2O3has been examined using transmission electron microscopy. A new experimental approach to the study of solid-state reactions, which may be adapted for in-situ experiments for low-temperature systems, has been used in this study. In its present form, the technique involves heating a thin film of one oxide in the presence of a vapor of the second oxide. In the study of the growth characteristics of Ni-Al spinel phase, the orientation of the Al2O3substrate has been found to influence greatly both the structural and morphological aspects of the spinel growth. In particular, the topotactic relationship between the spinel and the alumina are very different for (0001) and {1120} substrate orientations. The very early stage of the kinetics of the spinel growth, in which the length and the width of the spinel particles are only a few hundred angstroms, is illustrated with the results obtained from the re-heating experiments. The structure of the spinel-alumina interface has also been studied using high-resolution electron microscopy. These results are discussed in relation to the different models proposed for the spinel-alumina phase transformation.

The Kinetics of Ni-Al Spinel Growth Using Rutherford Backscattering Spectroscopy

ABSTRACTUsing a planar thin-film specimen geometry, the growth kinetics of the spinel in NiOAl2O3 system has been studied with Rutherford backscattering spectroscopy. A thin-layer of Ni film is deposited by the electron-beam deposition technique onto single-crystal alumina substrates of different orientations including, (0001), {1120}, {1102} and {1100}. The subsequent heat-treatment in air then converts the Ni to NiO, thus producing a uniform layer of NiO with good adhesion between the NiO and the alumina. The kinetics of the Ni-Al spinel growth has been found to be different for different single-crystal substrate orientations. The kinetics behavior follows a parabolic growth-rate law for each orientation but shows a different reaction-rate constant. X-ray diffraction and transmission electron microscopy have been used as complementary techniques to confirm the phases that form at each stage of the heat treatment and the corresponding microstructures of the thin-film layers respectively.

A new approach to the study of solid-state reactions

Abstract A new technique for the study of solid-state reactions is described. The new approach uses a thin-film specimen geometry and is ideal for transmission-electron-microscopy studies of the same specimen before and after the Occurrence of solid-state reactions. The technique of illustrated by an investigation of the growth of Ni-Al spinel on, and into, an alumina substrate. Near-topotactic relationships and exact topotactic alignment of the spinel and the alumina were both found. The {111} plane of the spinel is parallel either to the (00011) plane of the alumina or to the {1120} plane of the alumina. Further heat treatment of such samples directly reveals the mechanism by which the spinel particles grow.

The Study of the Formation and Growth of Ni-Al Spinel Using a New, Thin-Film, Specimen Geometry

AbstractThe formation of nickel-aluminate spinel during the reaction between nickel oxide and alumina has been studied by a new approach which utilizes a thin-film substrate of alumina. It is found that the spinel grows in either near- or exact-topotactic alignment along the edges of the thin-film substrate. It also grows on and into the surface of the substrate, but the deviation from exact topotaxy then tends to be larger. The topotactic relationship is such that the (0001) of the alumina is parallel or nearly parallel to the {111} planes of the spinel with <1100> parallel to <110>. A second orientation relatioship is found where the {1120} of the alumina is parallel or nearly parallel to the {111} planes of the spinel; <1100> is again parallel to <110>. First order twin boundaries are found to be common in the spinel particles which have grown on the edge of the thin alumina substrate. Direct observation of the change in the orientation of the spinel twins with respect to the alumina substrate after reannealing is reported. The morphology of the various types of spinel particles is discussed briefly.