Effects of Antioxidants Particle Size on Oxidation Resistance of MgO-C Refractory

In this study, Si and SiC powder with a critical particle size ranged from 0.1 to 0.4mm was added into MgO-C refractory as antioxidants. At 1200°C in air atmosphere, oxidation weight losses of cylindrical specimens with additives were measured and the effective diffusion coefficient of oxygen in refractories was calculated from the results. Thus, the effects of antioxidants particle size on the oxidation resistance were researched. The result shows that the particle size of antioxidant has a considerable influence on oxidation resistance of material. The oxidation resistance of MgO-C refractories increased at first as the critical particle size of Si powder increased from 0.1 to 0.2mm and then decreased as the critical particle size increased up to 0.4mm, while the oxidation resistance of MgO-C refractories decreased as the critical particle size of SiC additives increased from 0.1 to 0.4mm. The minimum effective diffusion coefficients of oxygen in MgO-C refractories added by Si and SiC were 10.90 and 14.09cm2/min, individually.

Investigation of Oxygen Diffusion in Irradiated UO2 with MD Simulation

In this study, irradiated UO2 is analyzed by atomistic simulation method to obtain diffusion coefficient of oxygen ions. For this purpose, a couple of molecular dynamics (MD) supercells containing Frenkel, Schottky, vacancy and interstitial types for both anion and cation defects is constructed individually. Each of their contribution is used to calculate the total oxygen diffusion for both intrinsic and extrinsic ranges. The results display that irradiation-induced defects contribute the most to the overall oxygen diffusion at temperatures below 800–1,200 K. This result is quite sensible because experimental data shows that, from room temperature to about 1,500 K, irradiation-induced swelling decreases and irradiated UO2 lattice parameter is gradually recovered because defects annihilate each other. Another point is that, concentration of defects enhances the irradiation-induced oxygen diffusion. Irradiation type also has the similar effect, namely oxygen diffusion in crystals irradiated with α-particles is more than the crystals irradiated with neutrons. Dynamic Frenkel defects dominate the oxygen diffusion data above 1,500—1,800 K. In all these temperature ranges, thermally induced Frenkel defects make no significant contribution to overall oxygen diffusion.

Biophysical basis for convergent evolution of two veil-forming microbes

Microbes living in stagnant water typically rely on chemical diffusion to draw nutrients from their environment. The sulfur-oxidizing bacterium Thiovulum majus and the ciliate Uronemella have independently evolved the ability to form a ‘veil’, a centimetre-scale mucous sheet on which cells organize to produce a macroscopic flow. This flow pulls nutrients through the community an order of magnitude faster than diffusion. To understand how natural selection led these microbes to evolve this collective behaviour, we connect the physical limitations acting on individual cells to the cell traits. We show how diffusion limitation and viscous dissipation have led individual T. majus and Uronemella cells to display two similar characteristics. Both of these cells exert a force of approximately 40 pN on the water and attach to boundaries by means of a mucous stalk. We show how the diffusion coefficient of oxygen in water and the viscosity of water define the force the cells must exert. We then show how the hydrodynamics of filter-feeding orient a microbe normal to the surface to which it attaches. Finally, we combine these results with new observations of veil formation and a review of veil dynamics to compare the collective dynamics of these microbes. We conclude that this convergent evolution is a reflection of similar physical limitations imposed by diffusion and viscosity acting on individual cells.

Theoretical Evaluation of Oxidation Rate of Zr

ABSTRACTFirst principle calculations were performed to evaluate stress effect on the diffusion process of oxygen vacancy in ZrO2 film, and oxidation rate of Zr was evaluated by solving simple diffusion equations. Our calculation results have indicated that both the vacancy formation and migration energies of ZrO2 increase with increasing compressive applied stress. The energy increase causes a decrease in the diffusion coefficient of oxygen vacancy in ZrO2, leading to a decrease in oxidation rate of Zr. The stress effect on diffusion process may explain the experimental fact that Zr is oxidized in proportion to the cubic root of time.