RESUMEN
Irreversible adsorption, or heel buildup, negatively impacts activated carbon performance and shortens its lifetime. This study elucidates the interconnections between flow rate and the oxygen impurity of desorption purge gas with heel buildup on beaded activated carbon (BAC). Nine thermal desorption scenarios were explored, varying nitrogen purge gas oxygen impurity levels (<5 ppmv, 10,000 ppmv, 210,000 ppm (21 %)) and flow rates (0.1, 1, 10 SLPM or 1 %, 10 %, 100 % of adsorption flow rate) during thermal desorption. Results reveal that increasing purge gas flow rate improves adsorption capacity recovery and mitigates adverse effects of purge gas oxygen impurity. Cumulative heel increased with higher purge gas oxygen impurity and lower flow rates. In the least effective regeneration scenario (0.1 SLPM N2, 21 % O2), a 32.8 wt% cumulative heel formed on BAC after five cycles, while the best-case scenario (10 SLPM N2, <5 ppmv O2) resulted in only 0.3 wt%. Comparing the pore size distributions of virgin and used BAC shows that heel initially forms in narrow micropores (<8.5Å) and later engages mesopores. Thermogravimetric analysis (TGA) showed that oxygen impurity creates high boiling point and/or strongly bound heel species. TGA confirmed that higher purge gas flow rates reduce heel amounts but encourage chemisorbed heel formation in oxygen's presence. These findings can guide optimization of regeneration conditions, enhancing activated carbon's long-term performance in cyclic adsorption processes.
RESUMEN
During silicon crystal growth, oxygen, a well-known major impurity, affects the final silicon wafer's mechanical and electrical properties. This study focused on regulation of discharge of different concentrations of oxygen from the quartz crucible into the silicon melt while considering the crucible angular speed and the friction at the melt-crucible interface. The three-dimensional transient governing equations for heat transfer, fluid flow, and impurity transportation in the Czochralski (CZ) puller were solved numerically. The oxygen solvation equation representing the crucible to silicon melt was modified to evaluate the accuracy of oxygen concentration calculations during the CZ process. Experimental measurements using the Fourier-transform infrared (FTIR) technique were used to confirm the simulation results. The results demonstrate that the crucible angular speed affects the oxygen concentration near the crucible wall and therefore in the silicon ingot. The proposed modifications for evaluating oxygen concentration offer a more comprehensive understanding of the oxygen dynamics during the CZ crystal growth.
RESUMEN
Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides (TMDs). Here, the phenomenon of oxygen impurities trapping photoexcited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect-capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First-principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in TMDs, and facilitate defect engineering in relevant materials and device applications.