Positron annihilation preparation

The use of Positron Annihilation Lifetime Spectroscopy (PALS) technique to characterize porous carbon materials has been analyzed. Positron annihilation lifetimes have been measured in two series of petroleum pitch-based activated carbon fibers (ACF) prepared by CO2 and steam activation. Two lifetime components were found a short-lived component, Ti from 375 to 393 ps and a long-lived component, 1 2 from 1247 to 1898 ps. The results have been compared to those obtained by Small Angle X-Ray Scattering (SAXS) and N2 and CO2 adsorption at 77K and 273K respectively The correlation found demonstrates the usefulness of PALS to get complementary information on the porous structure of microporous carbons. 

The porous structure of PHEMA hydrogels prepared under a range of conditions has been examined using three distinct probes. Positron annihilation lifetime spectroscopy and Xe NMR are sensitive to pores in the range 0.1-10 nm, while H NMR of water within the hydrogel provides information on a range of sizes up to several microns. PHEMA samples were prepared in solution with from 5-30 wt. % of water in the polymerization mixture. Below 30 wt. % the water exists in nanometer-sized pores, and above this a substantial proportion reside in micron sized pores. The PALS and Xe NMR results support the existence of relatively hydrophobic domains of sub-nanometer size. 

One of the main tasks of nuclear-reactor safety research is assessing the integrity of the reactor pressure vessel (RPV). The properties of RPV steels and the influences of thermal and neutron treatments on them are routinely investigated by macroscopic methods such as Charpy V-notch and tensile tests. It turns out that the embrittlement of steel is a very complex process that depends on many factors (thermal and radiation treatment, chemical compositions, conditions during preparation, ageing, etc.). A number of semi-empirical laws based on macroscopic data have been established, but unfortunately these laws are never completely consistent with all data and do not yield the required accuracy. Therefore, many additional test methods are needed to unravel the complex microscopic mechanisms responsible for RPV steel embrittlement. Our study is based on experimental data obtained when positron annihilation spectroscopy (PAS) and Mdssbauer spectroscopy (MS) were applied to different RPV steel specimens, which are supported by results from transmission electron microscopy (TEM) and appropriate computer simulations. 

Lanthanum laurates were prepared as in Ref. [2]. The spectra of the angular correlation of annihilation photons (ACAP) were measured in a standard long-slit geometry using 22Na radioactive isotope as a positron source. 

The samples for PALS experiments were prepared in the sandwich configuration (sample-positron source-sample). During the preparation it was formed into pastille (mechanically pressed to about 50 MPa) and placed in the argon pressure chamber. The other part of the same sample was mechanically compressed. The MCM-41 sample was put into a small tube, closed by two movable pistons sealed by 0-rings. The air from the container was evacuated to the pressure of about 0.5 Pa. Such a set was placed in the argon pressure chamber, so that mechanical pressure exerted by the pistons pushed by argon could be applied to the sample. The PALS spectra were collected in both cases in the pressure range 0.1-490 MPa. The positronium annihilation method for characterization of porosity of solids is based on the relation between ortho-positronium (o-Ps) lifetime and the size of free volume, in which o-Ps is trapped. The PALS spectra were processed as described in Ref. [9]. 

The results presented here show that the PHEMA hydrogels prepared in the presence of water contain a porous network on a number of length scales and with varying structure. On the angstrom level, substantial free volume has been identified by both Xe NMR and positron lifetime annihilation spectroscopy. It is likely that the free volume cavities detected by these two techniques exists within relatively hydrophobic, i.e. non-hydrated domains. On the micron level and larger a network of water-filled pores was identified by H NMR relaxation time measurements. It is this porous network that is responsible for the transport properties of PHEMA confirmed in numerous previous studies. The results are consistent with previous NMR studies of bulk-polymerized PHEMA. 

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