Diffusion in MgO

Oishi Y, Ando K (1984) Oxygen diffusion in MgO and AI2O3. Adv Ceram Structural Prop 10 379-395 Oishi Y, Kingeiy WD (1960) Self-diffusion of oxygen in single crystal and poly cry stalhne aluminum oxide. J Chem Phys 33 480-486... 

Oishi Y, Ueda H, and Terai R (1974) Self-diffusion coefficient of oxygen and its relation to viscosity of molten silicates. IntT Congr Glass, 10th ed. Ceram Soc Japan 8 30-5 Oishi Y, Ando, K, Kurokawa H, Hiro Y (1983) Oxygen self-diffusion in MgO single crystals. Cotmn Am Ceram Soc 66 C60-C62... 

Reddy KPR, Cooper AR (1976) Diffusion of oxygen in sapphire. Am Ceram Soc Bttll 55 402 Reddy KPR, Cooper AR (1982) Oxygen diffusion in sapphire. J Am Ceram Soc 65 634-638 Reddy KPR, Cooper AR (1983) Oxygen diffusion in MgO and a-Fc203. J Am Ceram Soc 66 664-666 Reddy KPR, Oh SM, Major LD Jr, Cooper AR (1980) Oxygen diffusion in forsterite. J Geophys Res... 

Traesdell AH (1974) Oxygen isotope activities and concentration in aqueous salt solutions at elevated temperatures Consequences for isotope geothermometry. Earth Planet Sci Lett 23 387-396 Tsuchiyama A, Kawamura K, Nakao T, Uyeda C (1994) Isotopic effects on diffusion in MgO melt simulated by the molecular dynamics (MD) method and implications for isotopic mass fractionation in magmatic systems. Geochim Cosmochim Acta 58 3013-3021 Tsuneyuki S, Matsui Y (1995) Molecular dynamic study of pressure enhancement of ion mobilities in liquid silica. Phys Rev Lett 74 3197-3200... 

Vocadlo L, Wall A, Parker SC, Price GD (1995) Absolute ionic-diffusion in MgO - computer calculations via lattice-dynamics. Phys Earth Planet Int 88 193-210 Watson GW, Parker SC (1995) Dynamical instabilities in a-Quartz and a-Berlinite A mechanism for amorphization. Phys Rev B-Cond Mat 52 13306-13309 Watson GW, Parker SC (1995) P-Quartz amorphization - a dynamical instabihty. Phil Mag Let 71 59-64. Watson GW, Parker SC, Wall A (1992) Molecular-dynamics simulation of fluoride-peiovskites. J Phys-Cond Mat 4 2097-2108... 

Ando, K., Oishi, Y, Koizama, H., and Sakka, Y, Lattice defect and oxygen diffusivity in MgO stabilized ZrOj, J. Mater. Sc. Lett., 1985, 4, 176-80. 

Para-selectivity for a wide variety of ZSM-5 preparations of comparable activity are shown in Figure 12. These data include results for unmodified HZSM-5 s of varying crystal size as well as chemically modified HZSM-5 s. Since the activity of these catalysts is nearly identical, these data clearly establish the major role of diffusion in the para-xylene content of the xylenes produced in TDP. We have examined in more detail the effect of the concentration of one of these chemical modifiers, MgO. 

It is seen that the intracrystalline MgO induces pore blockage in a fraction of the pore system and alters the porosity as well as D0, and/or r with the latter factors contributing most to the reduced diffusivity. In contrast, the coke modifier appears to affect mainly the surface-to-volume ratio and suggests that the effective surface area, number of available entrance ports, is reduced by two orders of magnitude. 

Figure 3-21 Calculated diffusion profiles for a diffusion couple in a ternary system. The diffusivity matrix is given in Equation 3-102a. The fraction of Si02 is calculated as 1 - MgO - AI2O3. Si02 shows clear uphill diffusion. A component with initially uniform concentration (such as Si02 in this example) almost always shows uphill diffusion in a multi-component system.

Diffusive dissolution of MgO-rich olivine and diffusion profiles MgO is the principal equilibrium-determining component and its diffusion behavior is treated as effective binary. Consider the dissolution of an olivine crystal (Fo90, containing 49.5 wt% MgO) in an andesitic melt (containing 3.96 wt% MgO) at 1285°C and 550 MPa (exp 212 of Zhang et al. 1989). The density of olivine is 3198 kg/m, and that of the initial melt is 2632 kg/m. Hence, the density ratio is 1.215. To estimate the dissolution parameter a, it is necessary to know the interface melt... 

Solution Because the profile is very short, and because diffusion of MgO in basalt is rapid (typical diffusivity is 10 fim ls), the time required to produce such a profile is about x lD < 40 s. Because even eruption time in conduit is longer than 40 s, the profile most likely formed due to growth of olivine during quench of the MORB sample in seawater. 

Equation (7.87) has far-reaching ramifications and basically predicts that in binary ionic compounds, diffusion-controlled processes are determined by D Ambh which is turn is a function of the individual component diffusivities. For most oxides, however, there are typically orders-of-magnitude differences between the diffusivities on the different sublattices (see, e.g.. Fig. 7.2). Consequently, with little loss in accuracy, Dambi can be equated to the slower diffusing species. For instance, in MgO, T>Mg2+ > Dqi and T>ambi ... 

Ando K, Kurokawa Y, Oishi Y (1983) Oxygen self-diffusion in Fe-doped MgO single crystals. J Chem Phys 78 6890-6892... 

Yoo HI, Wuensch BJ, Petuskey WT (1985) Secondary ion mass spectrometric analysis of oxygen self-diffusion in single-crystal MgO. Adv Ceram Stractuial Prop 10 394-405 Young ED, Rumble D (1993) The origin of correlated variations in in situ 0/ 0 and elemental concentrations in metamorphic garnet from sontheastem Vermont, USA. Geochim Cosmochim Acta... 

The co-ordination state of Co ions in MgO-CoO high area solid solutions has been investigated by means of diffuse reflectance spectroscopy. Co ions in tetrahedral co-ordination have been detected and discussed their presence appears very strange because of the octahedral nature of the cationic sites in the MgO lattice. The formation, upon oxygen adsorption, of dioxygen O2 adducts on surface Co ions emerging on (100) faces and corners of MgO-CoO microcubes has been studied by i.r. ... 

Surface states on MgO ° and CaO ° have been studied by u,v.— vis. and i.r. diffuse reflectance spectroscopy. The effect of outgassing (773—1073 K) is to develop u.v. absorption bands in the range 30 000—50 000cm . ° These bands have been attributed to absorptions at surface oxide sites in different states of co-ordinative unsaturation, the lower energy band corresponding to lower coordination. A theory has been developed for the surface centre in MgO the defect consists of an electron trapped at an anion vacancy on a (001) surface. ... 

Now you can compare diffusion in stoichiometric oxides with diffusion in nonstoichiometric oxides. The first question concerns the diffusing species when two species are present. For many ceramics AE is large. For MgO, AI2O3, and B2O3, AHs is 600kJ/mol ( 6eV/formula unit). Thus... 

Diffusion in ionic materials occurs primarily by the movement of charged species. Therefore, the application of an electric field can provide a very powerful driving force for mass transport. There have been numerous studies on the effects of electric fields on transport phenomena. Several studies have been performed on the evaporation of alkali halides in the presence of an external field. These investigations showed that the application of an electric field enhanced the evaporation of the crystal species. Similar studies have been performed on oxide ionic conductors, including ZrOi and p-aluminas. However, only a few experiments have been performed on classical insulating oxides such as a-A Os and MgO (perhaps because they are insulators). 

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