Surface fluorine concentrations

Total surface fluorine concentration values (pg F/cm2) were determined using neutron activation analysis. 

The process of mathematical fitting is error-prone, and especially two different issues have to be considered, the first one dealing with the boundary conditions of the fitting procedure itself A pure diffusion process is considered here as the only transport mechanism for fluorine in the sample. A constant value for the diffusion constant D, invariant soil temperatures and a constant supply of fluorine (e.g. a constant soil humidity) are assumed, the latter effect theoretically resulting in a constant surface fluorine concentration for samples collected at the same burial site. In mathematical terms, Dt is influenced by the spatial resolution of the scanning beam, the definition of the exact position of the bone surface, which usually coincides with the maximum fluorine concentration, and by the original fluorine concentration in the bulk of the object, which in most cases is still detectable. A detailed description on... 

The electrochemical results can be correlated with total fluorine contents obtained by elemental analysis and surface fluorine concentrations by XPS given in Table 4. 

Most of these discussions regarding fluorine contamination of aluminum surfaces have focused on the conversion of aluminum oxide to fluoride or oxyfluoride. Evidence for similar conversions was included, and in extreme cases conversion to aluminum bonding quite similar to that in AIF3 was found. However, the poor adhesion of the samples skipping the O2 plasma treatment is related not to the fluorine contamination as such, but rather to the carbonaceous nature of the adsorbed materials, which is subjected to the plasma polymerization of TMS. Oxygen plasma cleaning removes this carbonaceous component, while the surface fluorine concentration is enhanced. 

Arunyadej and Mitchell (1998) analysed the loss of surface fluorine in cotton fabrics treated with fluorocarbon by XPS data and showed that extended washing reduces the surface-fluorine concentration. Bulk-fluorine analysis of the treated fabric similarly shows a reduction in the fluorine... 

The Aerosol Fluorination process [42] (Aerosol) is operated on the principle that the substrate is absorbed onto the surface of fine sodium fluoride particles in the fluorination apparatus in which the fluorine concentration and the temperature increases along the length of the reaction vessel. A U.V. photo-fluorination finishing stage completes the perfluorination process which has the advantage that it is a continuous flow method. 

Fluorination of polyethylene surfaces leads to an increase in the surface energy, some degree of cross-linking and a reduction of the free volume of the polymer. All of these effects impart on the surface of the polymer a barrier that is very impermeable to hydrocarbon solvents. A blow-moulding process, in which a low concentration of fluorine in nitrogen is used as the blow-moulding gas, is used for the production of plastic fuel tanks for the automotive industry (Airopak , Air Products) [51]. Post-treatment of hydrocarbon surfaces with fluorine is an alternative technology and techniques for the surface fluorination of natural and synthetic rubber have been described [52]. 

As shown in Table 1, the application of APS to the F-contaminated Si02 surface causes a significant drop in the fluorine concentration. This result led us to study the removal of fluorine from the surface with water, which is the primary component in the APS solution. The results are shown in Table 3. 

Archaeological fragments of bones and teeth take up fluorine from the surrounding soil and accumulate it in their mineral phase when they are exposed to a humid environment. Geological time spans are needed for this process to reach equilibrium and for the fluorine distribution to become uniform. In cortical parts of long bone diaphysis, an initially U-shaped fluorine concentration profile can be observed, which decreases from the outer surface and the marrow cavity towards the inner parts of the bone and carries information on the exposure duration of the buried object in its shape. The time dependence of the profile slope is usually described in a simplified way by a diffusion model. The quantitative mathematical evaluation of these profiles may provide information on the exposure duration and the physical condition of the samples. Therefore, several attempts to use fluorine profiling as a dating method have been undertaken [3,39], The distribution of... 

Diffusion tends to equilibrate concentration differences between two reservoirs upon contact fluorine concentration profiles develop at the boundary of the two compartments as a function of time. Studies of the distribution of this trace element in archaeological samples such as bones, teeth or flints allow to gain some age information on the excavated objects of a burial site. The presented technique using beams of accelerated protons allows to measure fluorine diffusion profiles with an excellent space resolution. The surface exposure duration was deduced by the same method for Antarctic meteorites. 

CF, - plasma (Figure S ). As expected, CF4 plasma treatment leads to dramatic effects surface fluorination is seen to be very efficient in regimes "A" and "B , but the fluorine concentration drops rapidly at elevated Vs. The presence of hydrophobic CFX groups maintains high 0a values, as expected r decreases only slowly with increasing substrate bombardment, presumably due to the resulting surface cross-linking. 

Figure 15 reports experimental results obtained by emission spectroscopy on the relative evolution of the fluorine concentration in CF4—02 and SF6—02 mixtures in a rf diode reactor for various rf electrode materials (Al, Si, Ge) [55,65]. In the case of an electrode material inert with respect to the plasma (Al), addition of 02 to SF6 and CF4 induces, as expected, an increase of the fluorine concentration until the dilution effect lowers the F density. The lower F concentrations observed in the case of Si and Ge electrodes are due to the consumption of fluorine on these surface to form volatile SiF4 and GeF4. In the case of Si with SF6, from 0 to 40% 02 addition reaction is so fast that it consumes all the additional fluorine produced in the plasma. Above 40% 02, the F concentration increases gradually to... 

Adsorption of fluorine on the surface is supposed to obey Langmuir kinetics 0 the fraction of the surface covered with fluorine is smaller than one, with Ka (m3 s-1) being the adsorption rate constant per fluorine, and t] F the fluorine concentration in the gas phase near the surface. 

In the case of germanium, the formation of an oxyfluoride layer does not inhibit the etching in SF6—02 mixtures (only one slope), and for CF4—02 there is no dependency of the surface reactivity with the oxygen flux (null slope means Kd y>> Ka). Oxygen addition only provokes a modification of the fluorine concentration in the gas phase. Once more the energy brought by the ion bombardment explains these differences. 

An apparent upper limit on the depth of fluorination was observed even for reactions under ion-depleted conditions. This was attributed to an insufficient fluorine gradient from the surface of the polymer into the bulk. An increase in this gradient obtained by an increase of fluorine concentration in the plasma resulted in greater depths of fluorination suggesting a mechanism controlled by diffusion of fluorine species into the polymer. 

A high-density decoupled plasma is most important to make this approach work, first to achieve sufficient sidewall passivation in a comparatively low-pressure regime, second to obtain a high fluorine concentration and thus high etch rates, and third to achieve a low plasma potential, close to ground potential. With a plasma potential close to ground, ion energy flow to the wafer surface is con-... 

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