Fluxing action

Uses. Alkah metal and ammonium fluoroborates are used mainly for the high temperature fluxing action required by the metals processing industries (see Metal surface treatments Welding). The tendency toward BF dissociation at elevated temperatures inhibits oxidation in magnesium casting and aluminum alloy heat treatment. 

Since there are no volatile components this halophosphate phosphor is prepared with close to the stoichiometric amounts of SrHPO, SrCO, CaCO, BaCO, SrCl2, or NH Cl and EU2O2. The blend is fired under an atmosphere containing 1—2% hydrogen at 1100°C. A small excess of chloride provides some fluxing action and gives weU-formed crystals of apatite. The chlorapatites are dimorphous one modification is hexagonal and the other monoclinic. 

The uses of boric oxide relate to its behavior as a flux, an acid catalyst, or a chemical iatermediate. The fluxing action of B2O2 is important ia preparing many types of glass, gla2es, frits, ceramic coatings, and porcelain enamels (qv). 

One of the most important impurities, found particularly in the residual oils, is vanadium, occurring in the resultant ash as vanadium pentoxide. The fluxing action of the ash with which corrosion is normally associated and the marked difference in severity of attack at temperatures above and below the melting point of the ash again suggest molten salt induced attack. 

Without ion flux, action potentials cannot be generated in the regions covered with myelin. Instead, they occur only at breaks in the myelin sheath... 

Fig. 3 Mechanisms involved in the opioid ( r, 8, k, ORLi) and neuropeptide Y (Y2) receptor-mediated inhibition of exocytotic transmitter release. Following activation of the respective receptor and Gi/0 (1), three signal transduction pathways are possible, namely inhibition of voltage-dependent Ca2+ channels (2), opening of K+ channels (3), and a direct inhibitory effect on the vesicle release machinery (4). Glossary — > - leading to => - ion flux => - action potential Vm - membrane potential (+) and mean stimulatory and inhibitory effect, respectively.

Fluxes for Al components based on Na2B407/KBF4 mixtures were developed with the aim of dissolving AI2O3 films from component surfaces and have been used successfully in practice for decades. However, this achievement is an illustration of serendipity because they do not dissolve the oxide films but merely undermine them, (Jordan and Milner 1956). Few systematic studies of flux action... 

When fluxing action is needed during melting, and glass with high thermal expansion is undesirable, boric oxide is added forming borosilicate glasses, which have lower thermal expansion and improved resistance to attack by aqueous solutions because of lower sodium oxide contents. 

SEM microphotogaphs and EDAX scans of the cross section and outer surface of the slag deposit, illustrated in Figure 10, indicate the chemistry of the deposit is not uniform. The bulk of the fused material is rich in silica, low in iron, and virtually depleted of potassium. The outermost layers, no more than 2 to 3p thick, are very rich in iron and frequently also rich in calcium. On occasion, the outer surface is covered with small particulate, several microns in diameter, or undissolved cubic or octahedral crystals whose origin is pyrites. Similar formations have been observed in full-scale operation. The evidence indicates deposits form under axial symmetric flow conditions in the furnace by the fluxing action at the heat transfer surface of small particles, <8p in diameter, of decidedly different chemical composition and mineral source. Migration of the fly ash to the surface is by means of eddy diffusion, thermophoresis, or Brownian motion. 

Cryolite. Natural sodium aluminium fluoride, NajAlF m.p. 980 C sp. gr. 2.95. Because of its low m.p. and its fluxing action, it is used in the manufacture of enamels and glass and in the ceramic coatings of welding rods. Opal glass is often made from batches containing about 10% cryolite a similar preparation is sometimes used in white cover-coat enamels. 

The fluxing action of phosphorus is probably limited to the interface of the combustion and pyrolysis regions, so that any excess over the amount needed to stabilize the char is ineffective, and this excess is available for the formation of bromides without any loss of fluxing action. Since phosphorus oxides can effect only so much dehydration while the continually regenerated bromides can repeat the dehydrohalogenation process, phosphorus/bromine combinations would be more effective than phosphorus oxides in promoting char formation. 

A similar principle to pin transfer is used for the application of flux on the solder ball of FC components for DCA (See Fig. 40.14). The die is immersed into a thin film bath of flux. The flux depth allows only the balls to be coated so that, in effect, the solder balls become the pin, taking up the flux. The flux on the solder balls is transferred with the die to the circuit board, where it provides the tack function as well as fluxing action for the solder balls during the reflow step. 

Alkaline earths. They also provide fluxing action (but not as good as the alkalis). They are significantly better than alkalis for chemical durability, and for dielectric properties (i.e., the electrical conductivity of alkaline earth ions is less than alkalis they have a double ionic charge, thus are less mobile). 

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