Diffusion of Water and Oxygen

Doremus R. H. (1998) Diffusion of water and oxygen in quartz reaction-diffusion model. Earth Planet. Sci. Lett. 163, 43-51. 

Doremus RH (1983) Diffusion-controlled reaction of water with glass. J Noncrystal Solids 55 143-147 Doremus RH (1995) Diffusion of water in silica glass. J Mater Res 10 2379-2389 Doremus RH (1996) Diffusion of oxygen in silica glass. J Electrochem Soc 143 1992-1995 Doremus RH (1998) Diffusion of water and oxygen in quartz Reaction-diffusion model. Earth Planet Sci Lett 163 43-51... 

Figure 2 Schematic picture of the partially damaged coating system on a car body panel. Cracks after impact reach the primer. Diffusion of water and oxygen through the primer. (Picture drawn free after Granata [47].)...

Oxygen Availability in Degrading Films. A major difference between natural materials and starch-plastic or cellulose-plastic blends is that the hydrophilic and relatively permeable matrix of materials like wood and hydrated polysaccharide films allows diffusion of O2 and release of nutrients from sites at a distance from the invasion site. As colonization proceeds, pore enlargement occurs when the pore walls are degraded (8) or as the polymer matrix of amylose or PVA films is hydrolyzed (10.12). In contrast, the LDPE matrix supplies no nutrients, hinders diffusion of water and O2, and the pore diameter cannot be increased. The consequence of impermeability is that the sole means of obtaining O2 and nutrients is by diffusion through water-filled pores. 

TABLE III Diffusion Times for One- and Three-Dimensional Diffusion of Leucine and Oxygen Molecules in Water at Room Temperature... 

The needed apparatus and chemicals are placed in the dry box, and the box is closed and thoroughly flushed with dry oxygen-free nitrogen. When in use, it is kept under a small nitrogen pressure so that air will not diffuse into the box. A dish containing phosphorus pentoxide is often placed inside the dry box to ensure that moisture is removed, and if it is particularly important to remove all traces of water and oxygen, a dish containing the liquid sodium-potassium alloy may be placed inside the box. This alloy must, however, be handled with considerable care, for it will bum in air and will react explosively with water. 

In ambipolar diffusion of protons and oxygen ions, the two types of ions move in the same direction for charge compensation, and the net result is permeation of water. If we also have electronic transport, then the transport of hydrogen and oxygen ions may be de-coupled. They may move in the same direction or in opposite directions and at different ratios. 

In the previous section, an overview of the most common commercially available GDLs has been discussed. However, a great deal of research has been conducted on further modifications of commercial GDL or the preparation, at laboratory scale, of GDLs with particular properties. CP and CC processes produces material with low control of the porosity. Mainly, the preparation of non commercial GDLs aims to improve the porosity control in order to obtain a better diffusion of fuel and oxygen and for water management. 

Carbonation of concrete is the result of carbon dioxide gas diffusing into the concrete pores. Carbonation rates tend to be highest when the concrete is relatively dry since the pores contain little water to prevent entry of the gas but just sufficient to allow it to dissolve. Carbonic acid is formed which reacts with the free lime in concrete to form calcium carbonate and leads to a gradual fall in alkalinity from the surface inwards. Once the carbonation fi-ont reaches the steel reinforcement depassivation occurs and, in the presence of water and oxygen, corrosion can proceed. 

Examples of values of water and oxygen diffusivity determined in unidirectional composites are reported in Tables 12.4 and 12.5 respectively. Colin et al. (2005) showed that such models can also be nsed to predict oxygen dilfnsivity in composites. More recently, Roy and Singh (2009) showed that these models can be improved to take into acconnt physical discontinuities such as highly permeable fibre/matrix interface or libre/matrix debonding dne to oxidative shrinkage and erosion. 

Example 5.7 Estimation of Diffusivity of Hydrogen and Oxygen in Water Vapor Estimate the diffusivity of hydrogen in water vapor and oxygen in water vapor at 1 atm and 307 K and compare with experimental data in Table 5.4. 

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