Activation grain boundary diffusion

It can be seen drat the major difference lies in die activation energy being lower in the grain boundary. Data for a number of metals show that the activation energy for grain boundary diffusion is about one-half of that for volume diffusion. 

Here p is the density, a is the particle size, C and n are constants, Q is the activation energy for sintering, R is the gas constant and T is the absolute temperature, n is typically about 3, and Q is usually equal to the activation energy for grain boundary diffusion. 

Person 1 Calcnlate the self-diffusivity in polycrystalline silver (grain boundary diffusion), Dgb, at 500°C in m /s. What is the activation energy for this process in kJ/mol ... 

Stainless steel powder with a mean particle diameter of 50 mm has been compacted to a green density of 58% and sintered in pure H2. The resulting shrinkage measurements are given below. Published diffusion data for this stainless steel show that the activation energies are 225 kJ/mol for surface diffusion, 200 kJ/mol for grain boundary diffusion, and 290 kJ/mol for volume diffusion. Use the data below to determine the mechanism. 

Grain boundary diffusion is the dominant mechanism in polycrystalline tungsten below 2100 °C. For grain boundary diffusion, activation enthalpies between 377 and 460kJ mol were measiued. 

Grain boundary diffusion coefficients are about five orders of magnitude higher than volume diffusion in the same temperature range. The activation energy for grain boundary diffusion [54] is 363 25 kJ mol-1 - a remarkably similar value compared with that of volume diffusion. 

Examples of the temperature-dependence of diffusion in select silicate, oxide and carbonate systems are shown in Figure 23a for volume diffusion and Figure 23b for grain boundary diffusion. A comparison of diffusivities and activation data compiled in Table 2 (see Appendix) leads to the following general observations regarding diffusion behavior in crystalline-fluid (gas) systems. 

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