Deposition of diversely textured buffer pyrolytic carbon layer in TRISO coated particle by controlled manipulation of spouted bed hydrodynamics

Abstract

Predicting texture and physical properties of pyrolytic carbon (PyC) coatings produced by spouted bed coater/reactor has never been easy for specific yet critical hydrodynamic nature of the reactor. As a part of the TRistructural-ISOtropic (TRISO) coating for nuclear fuel applications, we have deposited PyC on surrogate zirconia particles by cracking of acetylene under variable static bed height, gas velocity and precursor concentration and characterized them thoroughly. From the analysis of samples, a correlation has been drawn between different experimental parameters and physical properties and texture of the said coatings. It has been shown that PyC with larger pores, higher degree of isotropic crystals and larger number of carbon atoms in lamellae are formed at higher spouted bed-height, whereas disordered crystals are produced at higher spouting gas velocities. Likewise, dense and hard coatings with smaller crystallite size are obtained with lower precursor concentration. The results have been adequately explained by invoking the droplet model. Our studies clearly shows that spouted bed hydrodynamics may be used to trap time frame of deposition, control surface temperature of spherical substrates and also the precursor gas concentration and thereby tailor the critical properties of the PyC coatings of TRISO coated particles.