Nanometric pores

Performing an accurate dosimetry is a prerequisite of any radiolysis study. However, all standard chemical dosimetry methods cannot be used in nanoporous media, as they all rely on known radiolytic yields that are expected to be modified by confinement. An alternative would be to use Monte Carlo particle transport code (i.e. MNCP, EGS4, Geant, Penelope) that allows a calculation of the deposited dose. PENELOPE (Penetration and Energy Loss of Positrons and Electrons) has especially proven to be efficient in microdosimetric calculations.This code can treat electron propagation down to 100 eV, for any material. Therefore, these simulations cannot handle sub-nanometric pore and do not take into account the fact that for certain materials pore diameter is smaller than spurs and/or track size. For nanometric to micrometric pores, the approximation usually proposed is to correct the dose by the mean density of the system... 

We have developed PS doped Eu " or Tb and Eu by a simple impregnation method. RBS and EDX/TEM analysis reveal a complete penetration of rare earth (Eu and Tb ) in the nanometric pores of PS. The PL study shows that Eu are diffused in Si nanocrystallites and occupies crystallographic sites in the matrix after annealing at 1000°C. We show that the luminescence of (Eu + Tb )/PS depends directly on wavelength excitation, which suggests that a process of excitation transfer occurs from Tb to Eu and to Si nanocrystallites when the radiative resonant transfer does play a key role. 

Essentially all these experimental methods apply when we deal with high-porosity multipore membranes, which are the majority of the cases. However, polymer brush decoration of single nanometric pores on macroscopic membranes has been examined lately [8]. A quite interesting method for the characterization of this system is the measmement of the ionic current that flows through the channel in an electrolyte solution under different electric potential differences. The current-voltage (TV) response of the system can be correlated to the conductance of the polymer-decorated channel and consequently to the effective free cross section of the pore-brush structure. 

Deeper investigations by FESEM enlightened such a stracture, as shown in Fig. 2 perforated waffles with nanometric pores and complex network configuration were present. The BET specific surface area of the as-prepared sample was 132 g [15]. 

Instead of blending them either by dispersing silica powders as fillers within PE, which is a well-developed technology with many applications, or by polymerizing ethylene within the pores of modified9 or unmodified silica,10 now the nanometric physical blending... 

Figure 15.4a shows a typical TEM image of the Pt wires, which clearly extend as dark stripes along the length of the mesoporous channels. The Pt wires (3nm in diameter) are consistent with the pore diameter of FSM-16 (2.7nm), and their length ranges extend to several hundred nanometres, reflecting the 1D channel structure of mesoporous silica templates (Figure 15.1). Moreover, the high-...

Nanometre hematite is produced by impregnating Si02 powder (with and appropriate pore size) with Fe(N03)3 solution, filtering the powder and heating at 600 °C for 3 hr (Morris et al., 1989). 

Small metal clusters can be incorporated into the pores of MCM-41 by encapsulating an organometallic compound by absorption and then decomposing it at low temperatures (2-300°C). Nanometre size Sn-Mo clusters have been made in this way. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Evidence considered in Sections 5.3.1 and 8.3 indicates that the C-S-H gel of calcium silicate or cement pastes has a layer structure, and that, together with a pore solution, it forms a rigid gel in which the pores range in size from macroscopic to enlarged interlayer spaces of nanometre dimensions. One can therefore define a water content only in relation to a specified drying condition. Three such conditions will be considered. 

Such a structure is termed macroporous with a typical average pore diameter of about 150 nm and a pore size range from several tens to several hundred nanometres. By comparison a gel resin is characterized by an apparent porosity of no greater than about 4 nm which represents the average distance of separation of polymer chains. This difference in structural characteristics of gel and macroporous resins is clearly evident when comparing Figures 3.2a and 3.2b. 

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