Influence of Foreign Elements

Traces elements have considerable influence on the quality of the W powder. They originate either from raw materials or are introduced diuing manufacture. They can, however, also be added intentionally prior to reduction, if specific powder properties are desired. 

Not only is the grain size affected by foreign element traces, but also size distribution, grain morphology, agglomeration, apparent density, and compactability of the W powder. 

Several foreign elements can stabilize the formation of metastable p-W, shifting the transition temperature from about 630 °C to above 800 °C [3.39]. Since all the stabilizers have a very high affinity to oxygen (Be, Al, Th, Sr, Zr, B, P), it was argued, that these elements form two-dimensional oxide compounds on the svuface of the P-W, and that the 

P-W a-W transformation may occur only after these compounds are decomposed by lydrogen [3.40], 

Elements or element combinations of industrial importance are discussed below. 

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Figure 7.7 The influence of foreign elements on the electrical resistivity of copper.

Although there are quite extensive studies concerning the effect of certain fission products on UO2 deposition in fused chlorides [25, 26], the influence of foreign elements contacting an uranyl-containing melt on the speciation of uranium has been but little studied. For example, some interaction of uranyl(VI) ions with metalhc platinum in a NaCl-KCl melt was noted by Smirnov and Skiba [27] but not by Stromatt [19]. The reaction between U02 and metallic molybdenum in this melt was also observed [26] but the mechanism of the process was not investigated. 

Influence of foreign elements introduced into a non-stoichiometric binary solid... 

To illustrate what preceded, we will treat in detail the case of the coalescence of the titanium dioxide (anatase) in the presence of water vapor and of oxygen. We will mention the results obtained for the anatase in the presence of oxygen and hydrogen chloride, with or without water vapor, and we will approach, in connection with ceria, the influence of foreign elements. These examples will enable us to see how the influences of the intensive constraints are introduced into rates of coalescence via their influences on the reactivity. 

The analytical determination of the foreign element c ncentration and distribution in tungsten is linked with highly sophisticated equipment (Auger spectroscopy, secondary ion mass spectrometry) and with complicated pretreatment of the samples. This may be one reason why our knowledge about foreign element influences is still quite incomplete. 

It is at least possible that many of these strongly held foreign elements provide adsorptive bonds. Support for this view is found in the fact that specific adsorptive and catalytic powers have been traced to the presence of noncarbon atoms—oxygen, iron, and nitrogen. The influence of noncarbon atoms may extend to adjacent carbon atoms in a manner analogous to the way in which a polar group introduced into an organic compound affects the chemical properties of remote atoms. 

Optical properties of dielectrics can be modified by incorporating nanosize clusters of foreign materials. Recently Si nanoclusters were shown to excite rare-earth element Er in the silica glass host [1,2]. The favorable effect of Si nanoclusters on the photoluminescence of Er in oxidized porous silicon (OPS) was also demonstrated [3], In silica hosts doped with Si nanoclusters it was shown that the excitation energy can be transferred from nanoclusters to Er ions located in a silicalike environment near the clusters. Nowadays, there is a principal interest to incorporate Er ions inside clusters due to influence on the excitation process. 

Figure 8. A) Influence of crystallite size (D) on electrical resistance for an SnO -based sensor in dry air ( ) and in air containing 800 ppm H (/ g) at 300 °C (elements sintered at 400 O B) Changes in SnO crystallite size as a function of calcination temperature for pure SnOj samples and Sn02 impregnated (5 %) with o,sides of various foreign metals [34] (with permission from Elsevier, Amsterdam)...

Some of the properties of copper are especially important for its utilization for electrical purposes. Among metals, only silver has better electrical and thermal conductivity. The electrical properties of copper, conductivity and resistivity, are highly influenced by low contents of some foreign elements while others are less harmful. This is shown in Figure 7.7. 

From an analytical point of view, the surface contamination adds foreign elements (C, O) to the composition of the surface of interest. More troublesome, it may alter the apparent concentration ratio of elements constituting the solid surface itself as the signal of interest is screened to an extent that depends on the electron kinetic energy. Furthermore, the contamination overlayer may differ, particularly in thickness, between the standards used for sensitivity factor determination and the unknown samples. The influence of the organic overlayer on the relative error in the surface composition of a model compound and its dependence on the line separation is illustrated in a recent paper. ... 

The adsorbed species play the part of foreign or doping elements on the surface of the solid. Thus, the first adsorbed species involve modifications of the later adsorption and influences the amount and state of the adsorbed species. 

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