Paul J. Jorgensen

High temperature transport processes in lithium niobate

Abstract The electrical conductivity of single crystal lithium niobate (LiNbO 3 ) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to P o 2 − 1 4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons. Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO 3 to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures. Thermoelectric measurements indicate that LiNbO 3 at low oxygen partial pressures is n -type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO 2 a is 4 × 10 17 cm 3 with a mobility of 1.7 cm 2 V sec. The diffusion of oxygen in LiNbO 3 was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O 18 /O 16 isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10 −6 exp ( −29.4 kcal mole −1 RT )cm 2 sec . Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li 2 O activity indicate that the ionic conduction is caused by transport of lithium ions.

Microstructural changes in SmCo5 caused by oxygen, sinter-annealing and thermal aging

Abstract The solubility of oxygen in SmCo 5 at 1125 °C (sintering temperature) in excess of the 800 °C solubility was determined to be 3500 ± 500 ppm. The source of oxygen during sintering is the samarium oxide subscale. Submicron oxide particles precipitate within SmCo 5 grains on cooling from the sintering temperature. Oxidation also causes depletion of samarium and precipitation of Sm 2 Co 17 particles within SmCo 5 grains. Both inclusions may be sources of domain wall nucleation. The oxide inclusions can be removed by a thermal aging treatment that collects the oxide into a few large grains outside the SmCo 5 grains by a solution/reprecipitation mechanism involving grain boundary transport of samarium. The Sm 2 Co 17 inclusions are not affected by thermal aging, but they can be prevented from occurring by including excess samarium in the sintered compact to replenish samarium lost by oxidation.

Microstructure and growth kinetics of the fibrous composite subscale formed by internal oxidation of SmCo5

The oxidation of SmCo5 is an unusual example of selective internal oxidation in which the subscale formed consists of a composite microstructure containing samarium oxide fibers and β-cobalt. The oxide fiber size increases with oxidation temperature, and the orientation of the fibers is generally perpendicular to the subscale-alloy interface. The unusual structure results because of the high concentration of samarium in the intermetallic compound, coupled with a low solubility of samarium in metallic cobalt resulting in a subscale consisting of 39 vol pct oxide. Growth of the subscale follows the parabolic oxidation law, and the kinetics have been determined between 100 and 1125°C. The kinetics are too fast to be explained by lattice diffusion in either the oxide or the cobalt phases. Oxygen diffusion down the cobalt-oxide fiber interface appears to be the transport mechanism for this diffusion controlled process. The oxidation behavior of PrCo5 is identical with that of SmCo5.

Oxidation-controlled aging of SmCo5 magnets

Abstract The minimum irreversible magnetic aging characteristics of SmCo5 magnets can be calculated based on the kinetics of internal oxidation of SmCo5. Experimentally measured open-circuit remanent induction losses determined as a function of time showed excellent correlations with the calculated curves for temperatures up to 200 °C. Above 200 °C deviations occurred from the calculated aging curves based on short-term oxidation kinetics because of coalescence of the oxide fibers in the subscale. Measurement of internal oxidation kinetics at 300 °C for times comparable with the magnetic aging times again allowed accurate computation of the irreversible magnetic losses.

Final-Stage Sintering and Grain Growth in Oxides

The atomic transport processes that result in densification during the sintering of oxides are controlled primarily by volume diffusion or by grain-boundary diffusion. Sintering can be described phenomenologically in terms of diffusion of atomic species from internal surfaces (i.e., grain boundaries) to pores. The flux of atoms from the grain boundaries is a function of the appropriate rate-limiting diffusion coefficient and the chemical-potential gradient. The purpose of this paper is to show that changes in the chemical-potential gradient can account for the densification of oxides that have been sintered to theoretical density.