THE OXIDATION OF COLUMBIUM AT 850 C IN OXYGEN AT LOW PRESSURES (thesis)
BS>A study was made of the oxidation of Nb in purified O/sub 2/ at 850 deg C and pressures of 5 x 10/sup -5/, 1 x 10/sup -4/, and 5 x 10/sup -4/ mm Hg for time periods up to 12 hours. Oxidation at and air pressure of 6 x 10/sup -6/ mm Hg was also investigated. Rate determinations were made by a gravimetric method involving the accurate measurement of the deflection of a sensitive quartz helix. The weight-gain versus time curves consisted of an initial, linear portion, the length of which depended inversely on the oxygen pressure, followed by a region in which the oxidation rates decreased with time. Electron micrographs of the oxidized surfaces and optical micrographs of both oxidized surfaces and metallographically prepared cross sections of the specimens indicated that oxidation proceeded initially by a process of nucleation and growth of small, geometrically shaped, well oriented crystals which were tentatively identified as an oxide phase. Single-crystal studies showed that the shapes and orientation of these initial growths depended upon the crystallographic face upon which they formed. The density of these geometrical growths was of the order of 10/sup 7/ to 10/sup 8/ per cm/sup 2/. Rapid solution of oxygen into the Nb accompanied the formation of the original crystal growths on the surface. It was found through microhardness measurements that oxygen, due primarily to grain-boundary diffusion, could reach the center of smallgrained, polycrystalline specimens rapidly enough so that essentially no macroscopic concentration gradient existed across the specimen. The result of this rapid diffusion and of local supersaturation was a process of internal oxidation due to precipitation of NbO at nucleation sites at or near grain boundaries. It was determined that the process of internal oxidation occurred at the temperature of oxidation and only partially during cooling to room temperature. It is believed that this study has produced the first reported observation of a process of internal oxidation in a pure metal. The deviations from linearity in the rate curves were correlated with the formation of significant thicknesses of NbO/sub 2/ , formed by oxidation of internally nucleated NbO crystals which had grown so as to protrude through the surface of the specimen. The linear rates observed in the initial stages of oxidation indicated that the oxidation rate was controlled by either an interface process or a steady-state situation involving diffusion through a thin film which maintained a constant thickness. (auth)
- Research Organization:
- Tennessee. Univ., Knoxville
- NSA Number:
- NSA-18-000566
- OSTI ID:
- 4171628
- Report Number(s):
- NP-13284
- Resource Relation:
- Other Information: Orig. Receipt Date: 31-DEC-64
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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Related Subjects
AIR
CONFIGURATION
COOLING
CRYSTALLIZATION
CRYSTALS
DENSITY
DIAGRAMS
DIFFUSION
EFFICIENCY
ELECTRON MICROSCOPY
EXPANSION
FILMS
GRAIN BOUNDARIES
HARDNESS
HIGH TEMPERATURE
IMPURITIES
LATTICES
MACHINE PARTS
MEASURED VALUES
METALLOGRAPHY
MICROSCOPY
MONOCRYSTALS
NIOBIUM OXIDES
NUCLEATE BOILING
OXYGEN
PHASE DIAGRAMS
PRECIPITATION
PRESSURE
QUALITATIVE ANALYSIS
QUARTZ
SENSITIVITY
SURFACES
TEMPERATURE
THICKNESS
WEIGHT