Metallic impurities and surface

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... 

To set down the necessary equations the composition of the solid including both metallic, nonmetallic impurities, and surface impurities must be determined or estimated when not known. It is also essential to know the composition and impurities present in the gaseous atmosphere... 

It is not within the scope of this chapter to provide a comprehensive discussion of silica gel chemistry. An excellent treatise is available (77). The parameters that most significantly affect bonding chemistries and solute retention properties are surface area, pore volume, pore diameter, trace metal impurities, and thermal pretreatments. Both Sander and Wise (90) and Sands et al. (91) have studied the effect of pore diameter and surface treatment of the silica on bonding reactions. Boudreau and Cooper (92) have studied the effects of thermal pretreatments at 180, 400, and 840°C on the subsequent chemical modification of silica gel, and showed that thermal pretreatment at temperatures >200°C can produce more homogeneous distribution of active silanols which are available for subsequent derivatization. 

Table 1. The rate parameters of the surface reaction evaluated hy fitting of the phenomenology (2) to the experimental dependencies of r on gas concentration for various sensors modified by individual metallic impurity and exposed to H2 and CO gases in the air of the relative humidity RH= 33 %.

Surface modifiers can be added to the talc by the talc supplier to improve or reduce various talc-polymer interactions. The choice and concentration of the modifier depends on the desired benefit in the talc composite. Silanes and titanates are used to improve the bond between the talc surface and the polymer. Other surface modifiers, such a glycols and sorbates, are used to improve dispersion of the talc in the polymer. Stearates, such as zinc or calcium stearate, are added to the talc or mixed into the compound to reduce die drool and lubricate die surfaces. Epoxies can be used to minimize interactions between talc and certain stabilizers or metal impurities. These surface-modified talc products are generally more expensive than untreated talc. The decision to use a surface-treated talc is based on cost-performance balance. Sometimes it is more cost-effective to add any additives during the compounding operation than have them added to the talc by the talc supplier. 

Solubility data were very confusing until it was found that traces of certain metal impurities and especially the presence of an amorphous or at least disturbed layer on the crystal surface caused variable results, especially at temperatures below 150 C. In 1952, Dempster and Ritchie (117) reported that siliceous dusts have a layer of high solubility that gradually blends into the solid core, which adsorbs basic dyestuffs (118). Alexanian (119) found by electron diffraction that quartz possesses a surface layer of amorphous silica about 100 A thick, which is removed by HF but is re-formed in ambient humidity. Waddams found that the quartz surface in water released mosaic silica, presumably as particles of colloidal size, since they scattered light (120). This was confirmed by Sakabe et al. (121), who found that in neutral or alkaline aqueous suspension, quartz released both soluble silica and colloidal particles of crystalline nature, 0.01-0.3 microns in size. Stober and Arnold (122) found that the amount of silica released was much more than a monomolecular layer, and that it decreased with successive changes of water. When quartz was intensively pulverized in water, the disturbed surface layer can amount to as high as 35%, with a specific surface area of 70 m g, and the solubility is increased from less than 10 to 70 ppm at 25 C (123). Paterson and Wheatley (124) made similar observations. 

The source material should be carefully cleaned and handled since, on heating, the volatile impurities and surface contaminants are the first materials to be vaporized. In some cases, the evaporant materials should be cleaned before they are used. Materials should be handled with metallic instruments since abrasive transfer can contaminate surfaces in contact with polymers. The source and source material can be outgassed and premelted prior to film deposition. 

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. 

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