Byunggi Kim

Effect of beam profile on nanosecond laser drilling of 4H-SIC

Laser processing has a great advantage of drilling of various materials due to its extremely high processing speed, particularly in the case that the deep drilling depth is required. For this reason, the through substrate via of semiconductors is now considered as one of the main target areas of laser processing. In this study, the numerical model, in which beam propagation is considered, is used to investigate the effect of beam profile on thermal ablation of 4H-SiC compared with experimental results. Considering the implementation of Bessel beam, near-infrared wavelength that was selected as 4H-SiC has good transparency to those wavelengths at room temperature. The main absorption mechanism was free carrier absorption, which indicates significant temperature dependence. The authors found that threshold fluence is dependent on the spot size of the beam due to heat conduction during several nanoseconds. In other words, resolution of the nanosecond laser ablation is limited no matter how small the spot size of the beam is. Also, carbonization induced by low fluence under the lattice melting temperature led to enlargement of the drilled crater. Our experimental results showed that Gaussian beam is a more efficient tool for deep drilling than Bessel beam because propagation of Bessel beam wavefronts is disturbed by opaque solid materials. Therefore, although the beam width of our Bessel beam was critically narrow (1.5 μm), a crater with high aspect ratio was not obtained. As a consequence, this study gives experimental and simple numerical analysis on the mechanism of the nanosecond laser drilling process of 4H-SiC.Laser processing has a great advantage of drilling of various materials due to its extremely high processing speed, particularly in the case that the deep drilling depth is required. For this reason, the through substrate via of semiconductors is now considered as one of the main target areas of laser processing. In this study, the numerical model, in which beam propagation is considered, is used to investigate the effect of beam profile on thermal ablation of 4H-SiC compared with experimental results. Considering the implementation of Bessel beam, near-infrared wavelength that was selected as 4H-SiC has good transparency to those wavelengths at room temperature. The main absorption mechanism was free carrier absorption, which indicates significant temperature dependence. The authors found that threshold fluence is dependent on the spot size of the beam due to heat conduction during several nanoseconds. In ...

Numerical study of heat transfer and chemical kinetics of solar thermochemical reactor for hydrogen production

A solar thermochemical reactor is a device utilizing concentrated solar energy to conduct hydrogen gas production by two-step water-splitting by dissociation of a reactive material, such as zinc oxide (ZnO). Reactor design, heat transfer, and reaction kinetics contribute a significant portion to the achievement of high solar-to-fuel conversion efficiency. In this work, an investigation of an indirect-cavity type reactor design performance has been conducted by numerical simulation method by coupling the computational model of fluid flow, energy equation, discrete ordinate radiation, and species transport. The reactor consisted of a windowed cavity reactor with an array of five tubes containing the flow of the reactive material. Dissociation of ZnO in a steady state condition of the reactor has been assessed under 1,500 sun heat flux from quartz window. A parametric study has been performed for a variation of the particle’s mass flow rate, solar flux peak in, and reactor configuration. The cavity of the reactor was insulated by a ceramic and reflective material to reduce the conduction and radiation losses. Inert gas of Ar was injected into the tube as product carrier. Energy balance analysis and reactor efficiency calculation have been performed to analyze the reactor performance. The results showed that the thermal re-radiation through the window and thermal conduction through the cavity wall dominated the heat losses around 84 % in total. The best operating condition in this study was at a mass flow rate 0.05 g/s, peak-heat-in 2,000 kW/m2, and tubes configuration of the staggered-front dominant. Some recommendations to improve the research include changing the chemical reactant with other metal-oxide which has a lower reactivity, increasing the tubes number to absorb the solar irradiation, combining the metal-oxide decomposition with other processes which require less heat, and applying the special material in window side which can filter the high wavelength from going outside the reactor to decrease the re-radiation losses.A solar thermochemical reactor is a device utilizing concentrated solar energy to conduct hydrogen gas production by two-step water-splitting by dissociation of a reactive material, such as zinc oxide (ZnO). Reactor design, heat transfer, and reaction kinetics contribute a significant portion to the achievement of high solar-to-fuel conversion efficiency. In this work, an investigation of an indirect-cavity type reactor design performance has been conducted by numerical simulation method by coupling the computational model of fluid flow, energy equation, discrete ordinate radiation, and species transport. The reactor consisted of a windowed cavity reactor with an array of five tubes containing the flow of the reactive material. Dissociation of ZnO in a steady state condition of the reactor has been assessed under 1,500 sun heat flux from quartz window. A parametric study has been performed for a variation of the particle’s mass flow rate, solar flux peak in, and reactor configuration. The cavity of the re...