Direct percolation pathway

A combination of low-pressure chemical vapour deposition (LPCVD) and plasma-enhanced chemical vapour deposition (PEC VD) was used to create a new multilayer composite SiOx/poly(paraxyly 1 ene) material for hermetic sealing of miniaturised smart micro-electromechanical systems (MEMS) implants (Hogg et al., 2014). Tailoring the thickness ratio between the layers, the percolative pathway and thereby, the permeation for direct water exposure could be considerably reduced compared to conventional parylene-C single layers with the same thickness. 

The existence of percolating pathways in a system such as a piece of sandstone can be studied experimentally by recording the movement of fluid invading the pore structures. Thus in Figure 9.14 direct measurement of fluid invasion in a piece of sandstone is shown. The movement of the fluid out from the injection point creates a fractal structure which is related to the pore structure of the material. In Figure 9.14(b) data processing generates the numerical estimates of the fractal dimension of the fluid front. In the same way it can be shown that if one drops a drop of liquid onto a porous pharmaceutical tablet the fluid tvill move out to create a fractal front related to the pore structure of the tablet [31]. 

In the bilayer heterojunction devices, the donor-acceptor phases are separated from each other and can selectively contact the anode and cathode, whereas in the bulk heterojunction both phases are intimately mixed and there is no preferred direction for the internal fields of separated charges. Fig. 20. The electrons and holes are thus created within the volume having concentration gradient (diffusion) as driving force. The separated charges require percolated pathways and the donor-acceptor phases form bicontinuous interpenetrating network [123]. Bulk heterojunction devices are sensitive to the morphology in the blend [124]. Majority of... 

Control layers, such as those used to minimize animal intrusion, promote drainage, and control and collect landfill gas, are often included for conventional cover systems and may also be incorporated into ET cover system designs. For example, a proposed monolithic ET cover at Sandia National Laboratories in New Mexico will have a biointrusion fence with 1/4-in. squares between the topsoil layer and the native soil layer to prevent animals from creating preferential pathways, potentially resulting in percolation. The biointrusion layer, however, will not inhibit root growth to allow for transpiration. At another site, Monticello Uranium Mill Tailings Site in Utah, a capillary barrier ET design has a 12-in. soil/rock admixture as an animal intrusion layer located 44 in. below the surface, directly above the capillary barrier layer. 

Graphites with larger surface areas or greater porosities have a distinctly lower percolation threshold. It is assumed that the conductivity of a compound depends upon the structured agglomerates being sufficiently close to each other, or in direct contact above the percolation point, and on the continuous current pathways created thereby (14-15). 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

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