Marker wires

If samples of two metals widr polished faces are placed in contact then it is clear that atomic transport must occur in both directions until finally an alloy can be formed which has a composition showing die relative numbers of gram-atoms in each section. It is vety unlikely that the diffusion coefficients, of A in B and of B in A, will be equal. Therefore there will be formation of an increasingly substantial vacancy concentration in the metal in which diffusion occurs more rapidly. In fact, if chemically inert marker wires were placed at the original interface, they would be found to move progressively in the direction of slowest diffusion widr a parabolic relationship between the displacement distance and time. 

Any unsymmetrically placed wires, or marker wires, are to be disregarded entirely. Center wires are subject to the same stipulations that apply to symmetrical wires. 

When this system was studied over time, it was found that the marker wires move toward each other. This shows that the most extensive diffusion is zinc from the brass (an alloy of zinc and copper) outward into the copper. If the mechanism of diffusion involved an interchange of copper and zinc, the wires would not move. The diffusion in this case takes place by the vacancy mechanism described later, as zinc moves from the brass into the surrounding copper. As the zinc moves outward, vacancies are produced in the... 

Other items such as tin snips, regular pliers, needle nose pliers, wire cutters, tin snips, razor knife, scissors, rulers, T-squares, Sharpie markers, wire connection crimpers, and wire cutters will be needed. Hand punches and reamers are also used. 

Fig. 3.10 Inverted x-ray radiograph of the postweld position of a lead marker wire in the (a) plan view and (b) side view. Note the ° through-material thickness traces of the lead wire in the side view. The initial placement of the lead wire can be observed...

Diffusion couples are experimental setups for determining the mechanism for the formation of the solid products. In a diffusion couple two solid reactants are pressed together with an inert net of marker wires made, e.g., of platinum between them. The diffusion couple is kept at the reaction temperature for a while, after which the spatial distribution of the reaction products with respect to the marker wires is... 

When two generalized solid oxides, AO and B2O3, react to the spinel AB2O4, the atoms move from the solid reactant lattices to the sites of the solid product. The ions A and B can be any main group or transition metal ions. As a specific example these are here taken to be CoO and Fe203, which form only one compound, cobalt ferrite (CoFe204). The marker wires end up somewhere in the spinel area or on the boundary of this phase, depending on how fast the ions of the two reactants are able to diffuse. Some conceivable diffusion mechanisms are in their pure form ... 

Generally, highly charged cations do not diffuse as well as cations with a smaller charge. The mechanism proposed here supposes that the trivalent iron ions are temporarily reduced to divalent iron, which is then assumed to be able to diffuse. The cobalt ions in this model are expected to remain stationary. The spinel is now formed only at the left-hand boundary between the spinel and the divalent oxide (CoO) hence the marker wires are found at the right-hand interface between the spinel and the trivalent oxide (Fe203) after the reaction. 

The bold-faced coefficients in these reactions show that after the reaction, the marker wires are in the interior of the spinel at a point that is one-third of the distance from the CoO interface. 

Platiaum and its alloys are also used as biomedical electrodes, eg, platiaum—indium wires for permanent and temporary pacemaker leads and defibrillator leads. Electrophysiology catheters, which contain platinum electrodes and marker bands, have been used to map the electrical pathways of the heart so that appropriate treatment, such as a pacemaker, can be prescribed. 

A marker is placed on the wire and a new marker is attached to the mouse pointer. We can place more markers if we want. Press the ESC key to terminate placing markers. The marker will change to the color the trace is displayed in Probe. On my computer, all Probe traces are displayed in black, so the marker is displayed in black. If your marker turns green, then the trace will be displayed in green on the Probe screen. 

Click the LEFT mouse button to place the marker. As you move the mouse away, you will notice that a marker is placed on the wire and that a second marker is attached to the mouse pointer ... 

Inert markers have been used to obtain additional information regarding the mechanism of spinel formation. A thin platinum wire is placed at the boundary between the two reactants before the reaction starts. The location of the marker after the reaction has proceeded to a considerable extent is supposed to throw light on the mechanism of diffusion. While the interpretation of marker experiments is straightforward in metallic systems, giving the desired information, in ionic systems the interpretation is more complicated because the diffusion is restricted mainly to the cation sublattice and it is not clear to which sublattice the markers are attached. The use of natural markers such as pores in the reactants has supported the counterdiffusion of cations in oxide spinel formation reactions. A treatment of the kinetics of solid-solid reactions becomes more complicated when the product is partly soluble in the reactants and also when there is more than one product. 

The specimen, most suitable for such measurements, is shown schematically in Fig. 1.8. The upper part of the specimen is used for comparison. To prevent the interaction of components A and B in this part, a thin barrier layer of some substance which does not react with both A and B under chosen experimental conditions is deposited. The position of the layer interfaces is measured at certain moments of time relative to the inert markers located at the initial interface between substances A and B and/or inside the ApBq layer. Microhardness indentations onto the specimen cross-section surface, thin wires and strips of chemically inert materials, bubbles of inert gases, etc., can serve as the markers (for more detail, see for example Refs 35, 124). 

Basic nanostructures encompass such simple and useful items as wires (a thin line of conducting atoms surrounded by an equally thin shell of insulating atoms), rods (which can be used in a variety of applications, from information storage and sensors to miniature diodes, cathodes and other integral components for electronics) and dots (small but exceedingly useful markers that can be used as atomic tracking devices). Other materials that will no doubt become extremely useful for self-assembly include nanocatalysts, which speed up chemical reactions. 

A brass (Cu-Zn) bar, wound with molybdenum wire, was plated with copper metal. The specimen was annealed in a series of steps, in which the movements of the molybdenum wires were recorded. The inert markers had moved from the interface towards the brass end of the specimen, which contained the fastest diffuser - zinc. This is now called the Kirkendall effect. A similar marker experiment had actually been performed by Hartley a year earlier while studying the diffusion of acetone in cellulose acetate (Hartley, 1946), but most metallurgists were not familiar with this work (Darken and Gurry, 1953). 

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