Sintering of Zinc Oxide 449 oxygen enters the lattice substitutionally. According to a theory by Lidiard,6 substitutional impurities diffuse in ionic crystals by forming complexes with vacancies on the same sublattice. One may therefore expect oxygen to diffuse into CaF2 by forming complexes with fluorine vacancies.One may further question whether the penetration is limited by the extrinsic or intrinsic diffusion rate of oxygen into CaF2 On the basis of Lidiard's theory, the activation energy for intrinsic diffusion, Qz, in CaF2 should be given by QX = U + (4) in which U is the activation energy for the mobility of complcxes and F is the formation energy of an anti-Frenkel defect. The intrinsic activation energy would be obtained when the oxygen concentration a t the penetration interface is much smaller than the intrinsic vacancy concentration. In the extrinsic limit, the diffusion activation energy, Qe, should be given by( e ) A. R Lidiard, "Impurity Diffusion in Polar Crystals,"Lidiard, "Impurity Diffusion in Crystals (Mainly Ionic Crystals with the Sodium Chloride Structure)," Phil. Mag.The extrinsic diffusion activation energy would be expected to control oxygen penetration if the oxygen concentration at the interface is much greater than the intrinsic fluorine vacancy concentration.Ure4 has determined that F = 65 5 kcal per mole in CaF2, so it remains to estimate U. If the mobility of complexes is limited by the ability of fluorine vacancies to circulate about the oxygen ion, one may expect U to be about equal to the 12 kcal per mole activation energy for fluorine vacancy mobility in CaF2.4 If this hypothesis is valid, Qt G 45 kcal per mole and QeThe experimental activation energy for oxygen penetration is 46 kcal per mole, consistent with the value estimated for the intrinsic-diffusion-limited case. The intrinsic vacancy concentration in the temperature range studied varies from 0.02 to 1.330/0.4 The oxygen concentration was 0.2y0 behind and about 0,0020/o in front of the penetration interface. At the highest temperature, even the oxygen concentration behind the interface satisfies the condition for intrinsic diffusion. Since no break from an Arrhenius relation was observed in the temperature range studied, the writers concluded that the penetration was intrinsic-diffusionlimited throughout. This conclusion was further supported by the fact that the penetration rate was the same in air and in oxygen in spite of the fact that the oxygen concentration behind the diffusion front was an order of magnitude greater for oxygen. 12 kcal per mole.Sintering studies have been conducted with single crystal spheres of zinc oxide in air at total pressures of lop3 to 1.0 atm over the range 1050' to 1250°C. Quantitative observations were made on the rate of growth of a neck between the spheres. No change in the distance between the geometrical centers of the spheres was observed. An analysis of the kinetic data shows that sintering was predominantly achieved by distillation through the surrounding gas phase.At 1150' th...

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A kinetic study of the reactions of ground state V, Fe, and Co with SO2 is reported. V, Fe, and Co were produced by the 248 nm photodissociation of VCl4, ferrocene, and Co(C5H5)(CO)2, respectively, and were detected by laser-induced fluorescence. V + SO2 proceeds by an abstraction reaction with rate constants given by k=(2.33 +/- 0.57)x 10(-10) exp[-(1.14 +/- 0.19) kcal mol(-1)/RT] cm3 molecule(-1) s(-1) over the temperature range 296-571 K. Fe + SO2 was studied in the N2 buffer range of 10-185 Torr between 294 and 498 K. The limiting, low-pressure third-order rate constants are given by k(0)=(3.45 +/- 1.19)x 10(-30) exp[-(2.81 +/- 0.24) kcal mol(-1)/RT] cm6 molecule(-2) s(-1). Co + SO2 was studied in the CO2 buffer range of 5-40 Torr between 294 and 498 K. This reaction is independent of temperature over the indicated range and has a third-order rate constant of k0=(5.23 +/- 0.28)x 10(-31) cm6 molecule(-2) s(-1). Results of this work are compared to previous work on the Sc, Ti, Cr, Mn, and Ni + SO2 systems. The reaction efficiencies for the abstraction reactions depend on the ionization energies of the transition metal atoms and on the reaction exothermicities, and the reaction efficiencies of the association reactions are strongly dependent on the energies needed to promote an electron from a 4s2 configuration to a 4s1 configuration.

The African School of Fundamental Physics and Applications, also known as the African School of Physics (ASP), was initiated in 2010, as a three-week biennial event, to offer additional training in fundamental and applied physics to African students with a minimum of three-year university education. Since its inception, ASP has grown to be much more than a school. ASP has become a series of activities and events with directed ethos towards physics as an engine for development in Africa. One such activity of ASP is the African Conference on Fundamental and Applied Physics (ACP). The first edition of ACP took place during the 2018 edition of ASP at the University of Namibia in Windhoek. In this paper, we report on the second edition of ACP, organized on March 7-11, 2022, as a virtual event.