Microplasma chemical reaction enhancement by laser modification of dielectric surface topography

Shin, Charles

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https://hdl.handle.net/2142/102472 Copy

Description

Title

Microplasma chemical reaction enhancement by laser modification of dielectric surface topography

Author(s)

Shin, Charles

Issue Date

2018-12-04

Director of Research (if dissertation) or Advisor (if thesis)

Eden, James Gary

Doctoral Committee Chair(s)

Eden, James Gary

Committee Member(s)

• Lee, Minjoo Lawrence
• Li, Xiuling
• Ruzic, David N.

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Microplasmas
• Plasmachemistry

Abstract

A newly developed etching technique, infrared laser ablation, for microplasma device fabrication is introduced. The ablated surface provides a topography that is distinct from the surface created by conventional techniques. Chemical, optical, and electrical experiments have been conducted to observe the difference in performance between devices fabricated with the conventional and with the new technique. Carbon dioxide (CO2) dissociation and ozone (O3) generation have been observed to verify the difference. Using the new laser ablation technique, CO2 dissociation energy efficiency has been increased from 13.5 ± 3.1% to 17.4 ± 5.7%. Furthermore, introducing 10% H2 (by vol.) into CO2 with the laser-ablated device has increased the energy efficiency to 23.5 ± 4.6%. In short, total energy efficiency increase of ~75% has been achieved by combining microplasmas with the new ablation technique and the H2 mixing. Optical emission spectroscopy observation shows that the CO2+ Fox-Duffendack-Barker system is dominant at low H2 flow rates, but the CO Angstrom bands start to dominate as H2 in the reactant mixture composition is increased. Using the residual gas analyzer (RGA), mass 30 (ethane or formaldehyde) and mass 46 (ethanol or formic acid) have been observed when H2 is mixed into the CO2 microplasmas. Calculated from the RGA signal, the maximum amount of ethanol generated is ~0.4 sccm when 100 sccm (80% CO2 and 20% H2 by vol.) is flowed into a single microplasma device (“chip”). Using the laser-ablated device, the efficiency of generating O3 has been increased by 7-11% depending on the flow rate. Microcavities within the microchannel generated by laser ablation have been observed, and the average cavity diameter has been calculated to be ~33 μm, with cavity density of ~300 mm-2. Intensified charge coupled device (ICCD) images of these cavities indicate that they discharge at lower applied voltage, while the observed optical emission intensity has been measured at ~2 times higher than typical microplasma regions at any given voltage. Furthermore, the laser-ablated device that contains cavities has higher electrical conductivities. Stark broadening has been measured, and the electron density has been calculated to be 1.2×10^16 ± 0.8×10^15 cm-3 and 1.1×10^16 ± 0.8×10^15 cm-3 for the laser-ablated and powder-ablated chips, respectively. Current-voltage (i.e. I-V) characteristics of laser-ablated chips show ~6% lower breakdown voltage. Also, higher current, compared to that of powder-ablated chips, at any given voltage has been observed for the laser-ablated chips. Owing to the higher surface area of laser-ablated chips, these electrical observations agree with increased field emission effect. As the dimensions of individual cavities will play an important role in further optimizing the plasma-surface interaction, production of uniform cavities has been attempted. Uniform truncated upside-down conical shapes with bottom and top diameters of ~150 μm and ~300 μm, respectively, have been fabricated. The laser ablation technique also has shown procedural advantages over the conventional technique. From a new microchannel design to a complete microplasma chip, micropowder ablation takes ~150 hours, whereas laser ablation technique requires only ~27 hours. Furthermore, no need for consumable chemicals, such as photoresist or silicone molds, makes the laser ablation technique a safer and more economical option as a surface ablation tool for microplasma production.

Graduation Semester

2018-12

Type of Resource

text

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http://hdl.handle.net/2142/102472

Copyright and License Information

Copyright 2018 Charles Shin

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