Fluorine content reduction

Ammonia solutions are significantly more efficient than ammonium acetate as washing reagents. The process of fluorine content reduction is related to subsequent hydrolysis of fluoride and oxyfluoride contaminants by ammonia. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

The most efficient washing of the hydroxide was achieved applying a three-step process using an ammonium carbonate solution as the first step, followed by an ammonia solution, and water as the final step. This washing process brings about a ten-fold reduction in the concentration of fluorine compared with laboratory and industrial experience, in which a 2-4 fold reduction in the fluorine content of tantalum or niobium hydroxides following a one-step washing process was obtained. 

To reduce fluorine content in (NFL MOg, the precipitate is washed. Use of water as a washing solution ensures a significant reduction of the fluorine content however, the process seems to be of low efficiency due to the relatively high solubility of ammonium peroxometalates. According to Belov et al. [512], solubility of the ammonium peroxometalates depends on the concentration of the ammonium salts. The solubility of ammonium peroxometalates (CM () ) in solutions of various ammonium salts can be... 

Additional confirmation of the approach can be found in the fact that mere thermal treatment of powder obtained from fluoride solutions by plasma chemical decomposition at 1000-1200K for 2-3 hours in air brings about a 100-300 fold reduction in fluorine content. Hence, the plasma chemical process and subsequent thermal treatment of the powder enables to obtain final products with fluorine contents as low as 10 2-10 3 % wt. 

Basically, the effect of a higher fluorine content on the polymer backbone or side chain will be the reduction of polymer chain-chain electronic interactions, which further results in a reduction of DE. This effect was observed by St. Clair et al.,3 but only on the polymer backbone. 

The wide-range frequency independence of these low DEs and the processability ofthese monomers suggest many potential applications. In particular, it is known that the addition of a fluorine-containing group to the polymer backbone will reduce the polymer chain-chain electronic interactions, which will result in a reduction of DE as reported by St. Clair et a/.13 However, the DE for the polymer from 6 (fluorine content 57%) is at most 0.13 below that for the polymer from 3 (fluorine content 46%), which may indicate that a minimum value has nearly been reached. 

In fluorous synthesis, the possibility for separation on a fluorous solid phase allows for reduction of the fluorine content in the tags. Fluorous resins offer a way to confront the high molecular weight problem resulting from fluorous tagging and therefore the problem of atom economy in fluorous-phase chemistry. 

Tests for chlorine, bromine, and fluorine content in fluxes are required as per IPC-TM-650, TM 2.3.33 and TM 2.3.35. The presence of these materials cause a reduction in the SIR and can accelerate corrosion of the PCB conductors and assembly solder joints. A test for chlorine and bromine is conducted by placing a droplet of flux on a silver chromate paper, which turns yellow or white if the halides are present. Presence of fluorine is determined by adding a droplet of purple zirconium alizarin to the flux region. A change in color from purple to yellow indicates fluorine is present. 

Another point of contention has been the extent to which, if any, SbFj is reduced to SbFs upon intercalation. Although chemical analyses have shown an F Sb ratio of 5 1 (Lll, M5), Sb Mossbauer measurements (B24) indicated partial reduction of Sb(V) to Sb(III). On the other hand, mass-spectral measurements as a function of temperature (S15) showed only SbFs, evolved in stages, with no fluorocarbons emitted at any time. The latter are usually an indication of partial reduction of the intercalant and fluorination of the graphite host. Wide-line, F-NMR chemical-shifts are consistent with either SbFj or SbFe, but not with SbFs, but the occurrence of fluorine exchange could produce minor amounts of trivalent species (FI 1) this point is thus still controversial, and will be alluded to again. 

Tungsten silicide deposited by DCS reduction contains much less fluorine than that by monosilane reduction, and the chlorine content is also low. Consequently, DCS is replacing monosilane in the CVD-WSix process because of good step coverage, good adhesion to polysilicon and Si02 and low resistivity. 

Etching has a profound effect on the surface chemistry of the fluoropolymers. Electron spectroscopy for chemical analysis (ESCA) also known as x-ray photoelectron spectroscopy (XPS) is a technique that has been used to study the chemical composition of surfaces. ESCA does not detect hydrogen, and elemental compositions exclude this element. Results of surface chemical composition of fluoropolymers have been summarized in Table 7.1. The consistent changes in surface composition of fluoropolymers because of treatment are a reduction in fluorine and/or chlorine content, and an increase in carbon and oxygen content. The treated PTFE surface is virtually comprised of carbon and oxygen and a small amount of fluorine. 

---