Silver Diffusion in Silicon Carbide Coatings

Abstract

We have studied the effect of microstructure on the diffusion of silver (Ag) in different silicon carbide (SiC) coatings in tristructural isotropic-coated particles. Silicon carbide was deposited at 13001-15001C by fluidized bed chemical vapor deposition. The SiC coatings have been heat treated at 12001-14001C for 240 h. Depending on the microstructure our results confirm that silver can diffuse by grain-boundary diffusion but that a high concentration of nano-and microporosity at the grain boundaries can strongly influence the diffusion process by increasing the diffusion coefficient by at least two orders of magnitude (from B10 À17 for standard coatings to !10 À15 m 2 /s at 12001C). We also show that the presence of excess silicon or the formation of long columnar grains in SiC does not increase the diffusion of silver as previously thought. Our results do not support the idea that silver diffuses through SiC due to the formation of defects produced by mechanical damage (nanocracks). Silver is aiding the recrystallization of SiC when heat treated at 14001C.

Effect of Thermal Treatment on Microstructure and Fracture Strength of SiC Coatings

“…It should be noted here that the fracture strength decreases less in coatings deposited at lower temperature (about 1300°C) than those deposited at higher temperature (1400°C–1500°C). This is consistent with previous study about high properties of SiC coatings deposited at low temperature, such as the hardness, Young's modulus, and resistance to the fission products …”

Microstructure of TRISO coated particles from the AGR-1 experiment: SiC grain size and grain boundary character

“…The grain boundary diffusion mechanism has been challenged by the work in Ref. [11], but is theorized as the dominant diffusion mechanism for Ag transport in SiC [8,9,[12][13][14]. Grain boundaries offer high diffusivity paths as compared to bulk material since the free volume, and associated energies, are higher at the grain boundary than within the crystals [14].…”

“…The lower diffusivity of metallic species in SiC with small grain sizes is attributed to the higher degree of tortuosity of the grain boundaries that hinder the diffusion of metallic species. This points to a grain boundary diffusion mechanism for 110m Ag in SiC [7][8][9]. However, diffusion of fission products through the SiC layer can be hindered by grain boundary engineering and by controlling the SiC microstructure during the manufacturing of the coating layers [6,9].…”

Tuning of the size and the lattice parameter of ion-beam synthesized Pb nanoparticles embedded in Si

“…The size distribution for sample 3 extracted from the 3D reconstruction of the volume is plotted in figure 2(d); an average radius of Pb NPs R TEM = 3.3 ± 0.2 nm is obtained, in good agreement with the SAXS results (R SAXS = 3.5 ± 0.3 nm). Moreover, the presence of a discontinuous 2D Pb thin layer at the interface between the crystalline Si substrate and the amorphous cap layer is observed, which may be attributed to a high mobility of Pb at grain boundaries of the interface [33].…”