Grain triple point

Fig. 1.16 Particle size and grain size in a nanostructured powder. Black points depict hydrogen molecules ( ) in pores between particles, and hydrogen atoms ( ) diffusing through a grain boundary network and gradually entering the grain interior black triangles point to triple points that are preferential sites for hydrogen atom accumulation...

The detection of other molecules, such as ammonia, requires the use of a porous catalytic metal. To obtain a gas response from the NH3 molecule, it is believed that active sites of triple points are required where the molecules are in contact with the metal, insulator, and ambient [30, 31]. It has been shown that gas species such as hydrogen atoms or protons also diffuse out onto the exposed oxide surface in between the metal grains [Figure 2.1(b)] [32, 33]. Furthermore, Lofdahl et al. have performed experiments that provide clear evidence that hydrogen atoms or protons also diffuse under the metal from the triple point [34]. The hollow structure of the metal surface facing the insulator has been revealed by Abom et al. [35]. 

Figure 17 (a) Grain boundary interface with amorphous region at the triple point, (b) EDX from the area x, showing mostly copper (oxide). 

The corrosion resistance of selected Si3N4 ceramics in NaOH is given in Fig. 40. The attack by bases is less pronounced than by acids. The extent of corrosion increases with increasing temperature and concentration of the bases [524], Also, in bases the intergranular films are not attacked as strongly as the triple points. In NaOH solutions often a linear dependence of the weight loss on time is reported. Materials which are less stable in acids are more stable in bases. This can be explained by the stability of the grain boundary phase [510],... 

Count the number of triple points in Figure 1.10 and deduce the ASTM grain size from this count. Compare with your answer to Problem 2A. 

The diffusion coefficients of the process controlling superplasticity may be enhanced or retarded by the addition of impurities or solute atoms or by the addition of secondary phases, normally used as sintering aids, which distribute along the grain boundaries and triple-point junctions of the grains. 

Grain Boundary Sliding and Triple-Point Separation... 

Ca+ ion is much smaller than Li atom. Some possibility of isolated Li incorporation probably as atomic form in grain boundaries, triple points, or defected zone in CaO film. 

As we will discuss in chap. 10, the consideration of polycrystals demands an analysis not only of the grain boundaries that separate different grains, but also of the points at which several different boundaries intersect. The simplest model of a triple junction is of the type indicated schematically in fig. 9.49. For our present purposes, the basic idea is to determine a condition for the stability of such triple junctions which will at once determine both the structure and energetics of these triple points. 

The problem is further complicated by the fact that there are three time scales to worry about sliding of grains with respect to each other, which in turn creates the negative pressure at the triple points that is responsible for the nucleation of the cavities, followed by the time needed to nucleate a cavity, and finally the growth and coalescence of these cavities — any one of which can be rate-limiting. 

Fig. 5.32 At low liquid contents, the liquid phase forms pockets at the triple points where three-grain boundaries meet. The shape of the liquid pocket is dependent on the dihedral angle. Reproduced with permission from [73]. Copyright 2009, Springer...

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