Hidemitsu Hayashi

Computer simulation study on filtration of soot particles in diesel particulate filter

A computational scheme is developed for simulating gas-particle flow in diesel particulate filter (DPF). It is composed of the lattice Boltzmann method for the gas and the Brownian dynamics method for soot particles suspended in the gas. In this scheme, the gas exerts a drag force on each particle, and on the other hand accumulated soot particles obstruct the gas flow by decreasing the gas permeability. Based on the analysis of the overall gas flow through DPF, the developed scheme is applied to simulating the gas-particle flow through a porous structure, which is fabricated by packing powder particles of several tens of micrometer size with the discrete element method so as to mimic the porous wall in SiC-DPF. At the initial stage of the simulation, the filtration efficiency of the porous structure in its clean state is evaluated as a function of diameter of the soot particle, and it is found out that the dominant collection mechanism transfers from the Brownian diffusion to the direct interception around the particle diameter of 300 nm, and the filtration efficiency has the minimum at around this diameter. A coupled simulation of the gas flow and the particle dynamics is executed repeatedly, updating the flow velocity and the gas permeability, until a considerable amount of soot particles accumulates in the porous structure. From this simulation, it is confirmed that the developed scheme can reproduce improvement of the filtration efficiency and deterioration of the back pressure caused by the accumulation of soot particles.

Nitrogen Ion Implantation and Thermal Annealing in 6H–SiC Single Crystal

The interrelationships among implantation-induced defect density, carrier activation rate, substrate temperature during nitrogen implantation and annealing in 6H–SiC have been clarified. Several defects, whose energy required for recovery of lattice damage depends on the substrate temperature during implantation, were examined. Although defect density was sufficiently low that it was undetectable by Rutherford backscattering spectrometry, the carrier activation rate was 3.3% under the condition that the implanted nitrogen density was 1.8×1019 cm-3. According to the first principles local-density functional calculation using the cubic SiC crystal model, the complex defect composed of interstitial carbon and substituted nitrogen, which produces a localized electronic state and a half-occupied level in the band gap, is considered to be one cause of the low carrier activation rate in nitrogen-implanted SiC.