Rates of oxidation of polycrystalline, (110), and (112) surfaces of iron have been measured at 24°, 100°, and 200°C by a manometric method using an initial oxygen pressure of about 1.3×10−2 Torr. At the lower temperatures the relative rates of oxidation are found to be polycrystalline, (110), and (112) in decreasing order. At 200deg;C the different single crystal surfaces oxidize at essentially the same rate. Logarithmic rate laws are obeyed reasonably well for oxidations up to 200°C, at which point the oxidation rate, after a time, obeys a parabolic law. Using this parabolic rate constant for the oxidation of polycrystalline iron at 200°C, together with previous microbalance data for the low pressure oxidation of iron at 350° and 400°C (1), an activation energy of 32 kcal. mole−1 is found for the formation of magnetite on iron. Surface areas of polycrystalline and (112) surfaces have been measured before and after oxidation using the BET method, employing krypton adsorption at 77°K. It is found that the number of surface sites for the adsorption of krypton is dependent on the nature of the adsorbent surface.

Nanocrystalline samples of highly pure lead oxide were prepared by the sol-gel route of synthesis. X-ray diffraction and transmission electron microscopic techniques confirmed the nanocrystallinity of the samples, and the average sizes of the crystallites were found within 20 nm to 35 nm. The nanocrystallites exhibited specific anomalous properties, among which a prominent one is the increased lattice parameters and unit cell volumes. The optical band gaps also increased when the nanocrystallites became smaller in size. The latter aspect is attributable to the onset of quantum confinement effects, as seen in a few other metal oxide nanoparticles. Positron annihilation was employed to study the vacancy type defects, which were abundant in the samples and played crucial roles in modulating their properties. The defect concentrations were significantly larger in the samples of smaller crystallite sizes. The results suggested the feasibility of tailoring the properties of lead oxide nanocrystallites for technological applications, such as using lead oxide nanoparticles in batteries for better performance in discharge rate and resistance. It also provided the physical insight into the structural build-up process when crystallites were formed with a finite number of atoms, whose distributions were governed by the site stabilization energy.

No abstract

No abstract

No abstract