Surface metallization, metallic impurities

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

The devitrification rate is extremely sensitive to both surface and bulk impurities, especially alkah. Increased alkah levels tend to increase the devitrification rate and lower the temperature at which the maximum rate occurs. For example, a bulk level of 0.32 wt % soda increases the maximum devitrification rate 20—30 times and lowers the temperature of maximum devitrification to approximately 1400°C (101). The impurity effect is present even at trace levels (<50 ppm) and can be enhanced with the addition of alumina. The devitrification rate varies inversely with the ratio of alumina-to-alkah metal oxide. The effect is a consequence of the fact that these impurities lower glass viscosity (102). 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

In either case the Pb contains numerous undesirable metal impurities, notably Cu, Ag, Au, Zn, Sn, As and Sb, some of which are clearly valuable in themselves. Copper is first removed by liquation the Pb bullion is melted and held just above its freezing point when Cu rises to the surface as an insoluble solid which is skimmed off. Tin, As and Sb are next removed by preferential oxidation in a reverberatory furnace and skimming off the oxides alternatively, the molten bullion is churned with an oxidizing flux of molten NaOH/NaN03 (Harris process). The softened Pb may still contain Ag, Au and perhaps Bi. Removal of the first two depends on their preferential solubility in Zn the mixed metals are cooled slowly from 480° to below 420° when the Zn (now containing nearly all the Ag and Au) solidifies as a crust which is skimmed off the... 

Figure 2. Segregation energy in layer Sp (p = 0 surface layer...) of a transition metal impurity of atomic number Z + 1 (d band-filling (Nj + l.l)e /atom, full curves (Nj + l)e /atom, dashed curve) in a BCC transition metal matrix of atomic number Z (d band-filling Nje" /atom) for various crystallographic orientations of the surface...

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Non-metallic impurities in liquid alkali metals play a major role in the corrosion of materials either by affecting metal solubilities, f orming spalli-ble corrosion products on the metal surface, promoting liquid metal embrittlement or bulk embrittlement of the surface or by sensitising the structure for further attack by other impurities e.g. O2. As in other corrosive environments the direction and magnitude of these impurity reactions... 

Pseudomorphism has less desirable consequences, and usually means are sought to suppress it. If the substrate has been scratched, ground or abrasively polished, or if it has been cold rolled or cold formed, the surface is left in a peculiar state. Cold working reduces the surface grain size, and produces deformed, shattered and partly reoriented metal. It may produce microcrevices between the deformed grains, and, with some processes, non-metallic impurities and oxides are embedded in the surface. The disturbed state of the substrate is copied by a pseudomorphic electrodeposit with several consequences (Fig. 12.7). One is aesthetic it has often been noted that almost invisible abrasion of the substrate develops as more prominent... 

The main metallic impurities that contaminate the primary powder, due to chemical corrosion of the retort and other metal parts of the reactor, are Fe, Ni and Cr. From this point of view, reactors that are equipped with larger retorts usually provide better purity due to a relatively low ratio between the internal surface of the wet metal parts of the reactor and the total volume of the melt. Recent investigations on the decreasing of Fe, Ni and Cr impurities during the sodium reduction process were performed by Li [591]. It was shown that one of the most effective ways to reduce contamination of the product is to reduce the duration of time K2TaF7 is present in the reactor. 

Since a metal is immersed in a solution of an inactive electrolyte and no charge transfer across the interface is possible, the only phenomena occurring are the reorientation of solvent molecules at the metal surface and the redistribution of surface metal electrons.6,7 The potential drop thus consists only of dipolar contributions, so that Eq. (5) applies. Therefore the potential of zero charge is directly established at such an interface.3,8-10 Experimentally, difficulties may arise because of impurities and local microreactions,9 but this is irrelevant from the ideal point of view. 

Binders (TbC) 671 Bipolar pulse conductivity detector (LC) 588 Bonded phases (GC) 125 crosslinked 126 estersils 125 nonextractable 126 siloxane 125 Bonded phases (LC) 324 carbon loading 335 cleavage of ligands 336 eluotropic strength (LSC) 382 endcapping 326 hydrophobicity 364 metal impurities 369 models for surface 337 physical characteristics 333, 366... 

TXRF is frequently used for contamination control and ultrasensitive chemical analysis, in particular in relation to materials used in semiconductor manufacturing [278,279], and metallic impurities on resin surfaces, as in PFA sheets [279,280], TXRF has been used by Simmross et al. [281] for the quantitative determination of cadmium in the four IRMM polyethylene reference materials (VDA-001 to 004). Microsamples (20-100 ig) from each reference material were transferred by hot pressing at 130 °C as 3 xm thin films straight on to quartz glass discs commonly used for TXRF analysis. The results obtained were quite satisfactory (Table 8.50). Other reports of the forensic application to plastic materials by TXRF have appeared [282], including a study of PE films by elemental analysis [283],... 

Unsaturated (drying) oils, like linseed oil, etc., will rapidly heat and ignite when distributed on active carbon, owing to the enormous increase in surface area of the oil exposed to air, and in the rate of oxidation, probably catalysed by metallic impurities [1]. A similar, but slower, effect occurs on fibrous materials such as cotton waste [2],... 

The violent decomposition observed on adding charcoal to cone, hydrogen peroxide is mainly owing to catalysis by metallic impurities present and the active surface of the charcoal, rather than to direct oxidation of the carbon [1], Charcoal mixed with a trace of manganese dioxide ignites immediately on contact with cone, peroxide [2],... 

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