Interdiffusion of metals

Interdiffusion of metals In silicon has also been studied by SXPS (39) idiere It was revealed that Au and Al Interact In very different ways with silicon. The Au-Sl Interface exhibits a strong chemical Interaction irtille the Al-Sl Interface shows much weaker Interactions. A fully reacted Sl-Cr Interface was found to be an effective barrier for Au-Sl Interdiffusion (40). 

Interdiffusion of metals into GaAs has also been studied with metals such as Pd (59) and Au (60). The effect of oxygen on the intermixing of the GaAs/Au Interface has also been studied (61) where oxygen was found to Inhibit interdiffusion. 

FIGURE 1.16 Bmarymetalliccompositions with minimum speed of interdiffusion of metals. 

We have been concerned hitherto with so much solvent that its amount for practical computations of D can be reckoned as infinite. Such a supposition limits the applicability of the equations given, for it will be much more usual to work with small amounts of diffusion media. The solutions now to be given will concern themselves wdth problems such as the following. A solute diffuses from a solution bounded between the planes x =0 x = h into a solvent bounded between the planes x and x = 1, The concentration-distance-time curves may be measured readily enough and have now to be interpreted so that the diffusion constants, 2), may be evaluated. The new solutions of Fick s law will apply to numerous cases of the interdiffusion of metals, and salts, so long as D does not depend on the concentration, and whenever the amount of metal or salt is limited. 

The most favoured experimental procedure in following the interdiffusion of metal pairs is to place a slab of each metal in contact, and heat them. The diffusion can be followed by X-ray or chemical analysis of thin layers near the origin, and the slabs may be regarded as of infinite thickness if the diffusion... 

DIFFUSION OF IONS. IN IONIC CRYSTALS AND THE INTERDIFFUSION OF METALS... 

The interdiffusion of gold and platinum is much slower than that of copper and nickel, and films formed by successive evaporation of the component metals were not brought to equilibrium by sintering at 200°C. Films formed by simultaneous evaporation were equilibrated by sintering... 

In some instances, to improve solderability, tin is deposited on nickel surfaces. In a short time, however, interdiffusion of the two metals results in the growth of an inter-metallic NiSn3 compound that is much less amenable to soldering. For tin over elec-trolessly deposited nickel surfaces, the interdiffusion results in pores in both films. Pores are to be avoided, of course, if conductivity and/or contact resistance is an issue. 

Diffusion must not, however, always be viewed as being a harmful phenomenon. In some cases it is most desirable, if not essential. Such is the case in welding (5), where diffusion ensures joining of the welded parts. Steel is often coated with tin to protect it from corrosion. In this case the formation, via interdiffusion, of the inter-metallic FeSu2 is the key for effective protection. 

These are depicted schematically in Figure 18.4 in the case of metal A deposited on metal B. Bulk diffusion, as noted above, is the transfer of B into A or A into B through the crystal lattice. This is characterized by the coefficient D in the figure. Defect path diffusion is the migration along lattice defects such as grain boundaries, characterized by the coefficient D in the figure. Ordered A B, possible phases are indicated between the metals. Finally, Kirkendall void porosity is indicated and will be expected to be present if the interdiffusion rates from one metal to the other are not equal in both directions. 

For interdiffusion of neutral metal atoms in alloys, the following relation, referred to as the Darken-Hartley-Crank equation, has been derived (Darken, 1948 Shewmon, 1963 Kirkaldy and Young, 1987) ... 

By depositing two (or more) different layers and annealing them, intermixing of the layers can lead to ternary and multinary compounds, although clear compound formation does not always occur. Thus, annealing (at 150°C, a relatively low temperature) ZnS-Cuj S and BbS-Cu S films resulted in extensive interdiffusion of the metallic elements but no XRD confirmation of solid solution formation [199]. On the other hand, Sb2S3-CuS layers converted fully to CuSbS2 at 400°C, which ex-... 

POWDER METALLURGY. Powder metallurgy (PM) embraces the production of finely divided metal powders and their union through the use of pressure and heat into useful articles. The temperatures required are below the fusion point of the principal constituent, and bonding depends on interdiffusion of the metal particles in the solid state. It is necessary to provide intimate contact between particles, hence reducing atmospheres are provided in the sintering process to prevent formation of oxide films Readily oxidized powders such as aluminum require special technique. 

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