Diffusion Fundamentals (E-Journal)
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Investigation of time-temperature relationships of surface segregations forming under internal adsorption of solved elements in α-Fe alloys, using Auger-spectroscopy
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Testing the (time)1/4 diffusion law of rehydroxylation in fired clays: evidence for single-file diffusion in porous media?
Get PDFFired clay minerals rehydroxylate after firing, a phenomenon that causes the long-term mass gain and expansion seen in archaeological ceramics. During rehydroxylation, fired clay minerals react with H2O to form structural hydroxyl (OH). Recent models propose that rehydroxylation is described by a quartic root (t1/4) time dependence, which has led to a dating method based on the process (RHX, rehydroxylation dating). The time dependence has been interpreted as an indication of a transport process of single-file diffusion of H2O molecules along restricted pathways in clay mineral, as theory predicts that the associated root mean square displacement is proportional to t1/4. However, dating trials have led to alternative models, and the process is still poorly understood. Here we outline currently outstanding problems found in different experimental studies, and propose novel ways of investigating the phenomenon. These include (i) gravimetric analyses using pure clay ceramics which are free of calcite and other minerals, and reducing cooling effects by using thin samples; (ii) highresolution nuclear magnetic resonance (NMR) with aluminum and silicon as well as with protons (hydroxyls, water) for exploring structure and dynamics, and pulsed field gradient (PFG) NMR for exploring water/proton diffusivities; (iii) IR spectroscopy, and micro-IR imaging to explore the spatial-temporal dependence of rehydroxylation of fired clay and (iv) structural investigations by high-resolution electron microscopy and X-ray diffraction
Grassmann variables and pseudoclassical Nuclear Magnetic Resonance
Get PDFThe concept of a propagator is useful and is a well-known object in diffusion NMR experiments. Here, we investigate the related concept; the propagator for the magnetization or the Green’s function of the Torrey-Bloch equations. The magnetization propagator is constructed by defining functions such as the Hamiltonian and Lagrangian and using these to define a path integral. It is shown that the equations of motion derived from the Lagrangian produce complex-valued trajectories (classical paths) and it is conjectured that the end-points of these trajectories are real-valued. The complex nature of the trajectories also suggests that the spin degrees of freedom are also encoded into the trajectories and this idea is explored by explicitly modeling the spin or precessing magnetization by anticommuting Grassmann variables. A pseudoclassical Lagrangian is constructed by combining the diffusive (bosonic) Lagrangian with the Grassmann (fermionic) Lagrangian, and performing the path integral over the Grassmann variables recovers the original Lagrangian that was used in the construction of the propagator for the magnetization. The trajectories of the pseudoclassical model also provide some insight into the nature of the end-points
Dehydration diffusion of B(OH)4-sodalite investigated by micro-Raman spectroscopy on single crystals and combined TG/IR on powders
Get PDFDehydration experiments were carried out on larger B(OH)4-sodalite single crystals, Na8[Al6Si6O24](B(OH)4)2, at temperatures 300°C, 350°C and 400°C. Profiles of relative intensity variations of B-(OH)/Si-O vibration could be measured by micro-Raman spectroscopy ranging typically between 5 and 40 μm into the interior of the crystals. Profile analyses reveal effective dehydration diffusion coefficients DOH between 10-11 and 10-9 cm2/s with an Arrhenius activation energy Ea ≈ 1.3 eV. H/D exchange experiments were realized at temperatures 50, 100 and 150°C in closed tubes. Profiles of OD/OH Raman intensities reveal effective H/D exchange diffusion coefficients DHD between 1.5∙10-10 and 4∙10-11 cm2/s with Ea ≈ 0.4 eV. The dehydration effect for crystalline powders was reinvestigated up to 500°C in TG experiments and by IR spectra. These data show that dehydration takes place forming NaBO(OH)2- and NaBO2-sodalite centered at temperatures 250°C and 410°C, respectively. It is discussed that diffusion coefficients DP could be estimated using the mass losses for the two stages of dehydration related to the experiments on the polycrystalline samples. Linear extrapolation of DOH, i.e. the diffusion coefficients obtained on larger single crystals, towards lower temperature well approximates DP using the dehydration effect obtained for crystalline powders between 140 and 180°C assuming spherical crystals of 0.1 μm diameters. H/D exchange could occur with proton/deuteron jumps through the four-ring windows of the sodalite framework. Larger ions or molecules predominantly pass through the six-ring windows which requires window opening
Competition between diffusion and advection may mediate self-repair of wax microstructures on plant surfaces
Get PDFCuticles are extracellular membranes covering the primary aerial parts of vascular plants. They consist of a multifunctional polymeric material with embedded soluble components, called waxes and serve as the interface between plants and their atmospheric environment, first of all protecting them from desiccation. Waxes are produced within the epidermal cells, then transported to the leaf surface and finally integrated into the polymer or deposited upon the cuticle. Remarkably, damaged wax layers may become repaired within a few hours. Base on an earlier hypothesis we present a theoretical framework explaining how waxes are transported through the plant epidermis by a combination of advection and diffusion. This combination suggests also a self-regulating repair mechanism, based on the assumption that intact cuticles induce an antagonistic equilibrium between advection and diffusion: whenever a wax layer is damaged, the equilibrium is disturbed in favour of advection, starting a repair process, which is intrinsically coming to an end after the cuticle has gained its original thickness
Transport of isopropanol in H-ZSM5 by impedance spectroscopy
Get PDFGas diffusion mechanisms and diffusion coefficients in microkinetic system of solid materials characterize the different performance of chemical and transport processes. These factors should be evaluated with respect to the range of diffusion coefficients based on the number of experimental methods. Here, impedance spectroscopy which is a powerful tool for understanding transport phenomena of gases in nanostructured porous solids is used. In this work, we developed a new approach to compare and evaluate diffusion coefficients of isopropanol in the zeolite H-ZSM5. Furthermore, FTIR spectroscopy was used to study the adsorption of isopropanol in H-ZSM5 at different temperatures to support the interpretation of diffusion regimes. The combination of EIS with FTIR enables to illustrate preferred interaction sites and their influence on transport processes. The activation energy for transport of molecules in the channels is also calculated. Results are compared to literature data and suggest improved applications of EIS to characterize transport processes
Astrocytes and diffusive spread of substances in brain extracellular space
Get PDFBrain function is based on communication between individual cells, neurons and glia. From a traditional point of view, neurons play a central role in the fast transfer of information in the central nervous system while astrocytes, major type of glia, serve as housekeeping elements maintaining homeostasis of the extracellular microenvironment. This view has dramatically changed in recent years as many findings ascribe new roles to astrocytes. It is becoming evident that astrocytes communicate with neurons via chemical signals released to the extracellular space (ECS). Astrocytes also have communication systems of their own, such as calcium waves that use gap junctions in combination with purinergic signaling through the ECS. Here we discuss yet another important role for astrocytes: that they regulate diffusion of signaling molecules and therapeutic agents in the extracellular microenvironment by contributing to the structural properties of ECS. There is a wealth of morphological data showing that each astrocyte is an exclusive occupant of a small volume of brain tissue, and that many fine astrocytic processes ensheathe neuronal processes and bodies. The functional significance of these unique morphological features is largely unknown with the exception of astrocytic coverage of synaptic formations. At the synapses, astrocytic processes play an active role by restricting neurotransmitter diffusion to the synaptic cleft and its immediate vicinity. Recent work suggests that astrocytic processes work in a similar fashion throughout the ECS and thus control the diffusive spread of substances over both short and long distances