Multi-Laser Technology for Clean Energy Applications: Remote Detection and Nano/ Micro Fabrication [abstract]

Only abstract of poster available.Track IV: Materials for Energy ApplicationsA new class of science and technology is being developed at Missouri S&T using multiple lasers to study the fundamentals of laser-material interaction and laser-based micro/nano fabrication. The new technology integrates two or more lasers, such as femtosecond laser, nanosecond laser and CW laser, with variable pulse durations, tunable wavelengths, and powers, allowing us to “manipulate and control” the chemical reaction path or ablation mode in laser-material interaction. Currently, the new technology is being developed to achieve “resonant” laser energy absorption for photo-dissociation of CxHy gases to generate “selected” free radicals that facilitate the formation and growth of diamond thin film in room temperature and open atmosphere. The multi-laser technique is also being used to fabricate miniature fiber sensors, cardiovascular stents, lab-on-a-chip, micro-lens array, and the cutting of silicon wafers. The new multi-laser technology can have broad applications in Clean Energy research including, for example, 1) in-situ monitoring of gas compositions in the hot-zone of a coal-fired power plant for improving efficiency and reducing pollutant emission; 2) remote detection and identification of trace pollutants (gases or particulates) for environmental management in fossil fuel-based power generation; 3) precision fabrication of micro-lens array for enhancement of the photovoltaic efficiency and microphotonic sensors for process control and monitoring in energy generation; 4) crack-free cutting of solar panels and micro welding for the assembly of fuel cells (laser welding has been identified by DOE as one of the key technologies for fuel cell manufacturing). Through the sponsorship of NSF, ARO, and AFRL, Missouri S&T is equipped with several state-of-the-art laser systems and the associated optics and instruments

Droplet Acceleration in the Arc

This paper simulates the acceleration of the droplet in the arc during gas metal arc welding process. After a droplet is detached from the electrode, it is accelerated in the high temperature and high velocity arc to the workpiece. The droplet is subjected to several forces, such as the arc plasma shear stress, arc pressure force, surface tension force, gravity force, and electromagnetic force. A comprehensive model is used to simulate the changes of droplet shape, temperature, and velocity during the acceleration in the arc. The transient interaction of droplet and arc plasma is through coupled boundary conditions, thus, no assumptions are needed to simulate the droplet acceleration. The simulated results were compared with the published experimental data and an agreement was found

Plasma Modeling for Ultrashort Laser Ablation of Dielectrics

In ultrashort pulse (\u3c10 ps) laser ablation of dielectrics, affected materials are first transformed into absorbing plasma with metallic properties and, then, the subsequent laser-plasma interaction causes material removals. For ultrashort-pulse laser ablation of dielectrics, this study proposes a model using the Fokker-Planck equation for electron density distribution, a plasma model for the optical properties of ionized dielectrics, and quantum treatments for electron heating and relaxation time. The free electron density distribution of the plasma within the pulse duration is then used to determine the ablation crater shape. The predicted threshold fluences and ablation depths for barium aluminum borosilicate and fused silica are in agreement with published experimental data. It is found that the significantly varying optical properties in time and space are the key factors determining the ablation crater shape. The effects of fluence and pulse duration are also studied

A Plasma Model Combined with an Improved Two-Temperature Equation for Ultrafast Laser Ablation of Dielectrics

It remains a big challenge to theoretically predict the material removal mechanism in femtosecond laser ablation. To bypass this unresolved problem, many calculations of femtosecond laser ablation of nonmetals have been based on the free electron density distribution without the actual consideration of the phase change mechanism. However, this widely used key assumption needs further theoretical and experimental confirmation. by combining the plasma model and improved two-temperature model developed by the authors, this study focuses on investigating ablation threshold fluence, depth, and shape during femtosecond laser ablation of dielectrics through nonthermal processes (the Coulomb explosion and electrostatic ablation). The predicted ablation depths and shapes in fused silica, by using (1) the plasma model only and (2) the plasma model plus the two-temperature equation, are both in agreement with published experimental data. The widely used assumptions for threshold fluence, ablation depth, and shape in the plasma model based on free electron density are validated by the comparison study and experimental data

Method and System for Far-Field Microscopy to Exceeding Diffraction-Limit Resolution

The bio-sample (e.g., a live cell) is labeled with a proper number of nanoparticles. Each nanoparticle is pre-co-doped with a controlled ratio of fluorophore donors and acceptors. Two laser pulses are applied to the bio-sample. The first laser pulse has a center wavelength near the peak of absorption spectrum of acceptors. The intensity of first laser pulse is adjusted such that FRET saturation occurs near the center of the focal spot. The focal spot of the first laser pulse is a diffraction-limited Airy disk that has the highest laser intensity in the center of the focal spot. The second laser has a center wavelength in the emission spectrum of acceptors and with a uniform intensity distribution throughout the focal spot. The fluorescence emission from acceptors after two laser pulses is from an area that is smaller than the diffraction-limited focal spot. Hence, a higher than diffraction-limit resolution is achieved

Effects of Current on Droplet Generation and Arc Plasma in Gas Metal Arc Welding

In gas metal arc welding (GMAW), a technology using pulsed currents has been employed to achieve the one-droplet-per-pulse (ODPP) metal transfer mode with the advantages of low average currents, a stable and controllable droplet generation, and reduced spatter. In this paper, a comprehensive model was developed to study the effects of different current profiles on the droplet formation, plasma generation, metal transfer, and weld pool dynamics in GMAW. Five types of welding currents were studied, including two constant currents and three wave form currents. In each type, the transient temperature and velocity distributions of the arc plasma and the molten metal, and the shapes of the droplet and the weld pool were calculated. The results showed that a higher current generates smaller droplets, higher droplet frequency, and higher electromagnetic force that becomes the dominant factor detaching the droplet from the electrode tip. The model has demonstrated that a stable ODPP metal transfer mode can be achieved by choosing a current with proper wave form for given welding conditions

Repeatable Nanostructures in Dielectrics by Femtosecond Laser Pulse Trains

Using the plasma model recent developed by the authors, this study predicts the existence of a constant ablation-depth zone with respect to fluence in femtosecond laser ablation of dielectrics, which has also been observed experimentally. It is found that the value of the constant ablation depth is significantly decreased by the pulse train technology. Repeatable nanostructures can be achieved with the parameters in the constant ablation-depth zone of a femtosecond pulse train, even when the laser system is subject to fluctuations in fluences

Microphotonic Harsh Environment Sensors for Clean Fuel and Power Generation

Track IV: Materials for Energy ApplicationsIncludes audio file (19 min.)Low cost, reliable, in-situ sensors are highly desired for advanced process control and lifecycle management in various power and fuel systems. Many energy generation processes involve harsh conditions throughout the operation that requires monitoring to assist in attaining and maintaining the goals of high efficiency and high environmental performance. General measurements of interest include temperature, strain, pressure, gas compositions, and trace contaminants/pollutants. Unfortunately, most existing sensors are incapable of operating directly in the harsh environment of typical power and fuel systems involving high temperature and high pressure with presence of particulates and corrosive atmosphere. Funded by DoE/NETL, our group has been developing various novel microphotonic sensors for monitoring physical and chemical parameters under hostile conditions. The demonstrated sensors include the miniaturized inline fiber Fabry-Perot interferometer (FPI) fabricated by one-step femtosecond laser micromaching, the long period fiber grating (LPFG) fabricated by CO2 laser irradiations, the fiber inline core-cladding mode interferometers (CMMI), and the LPFG coupled CMMI sensors. These sensors can be directly used for the measurements of various physical parameters such as temperature, pressure and strain in a high temperature (tested up to 1100 degree C) harsh environment are presented. In addition, when coated with a thin layer of gas sensitive thin film (e.g., doped crystalline ceramic nanofilm), they can be used for measurements of various hot gases such as hydrogen, carbon dioxide, carbon monoxide, and hydrogen sulfide in high temperatures. With demonstrated advantages of small size, lightweight, immunity to electromagnetic interference, resistance to chemical corrosion, high sensitivity, remote operation capability, robustness and dependable performance in a hostile environment, these microphotonic sensor may find broad applications for process control and optimization in various fuel/power systems such as coal gasification, advanced engines, oil/gas extraction, fuel cell operation, coal/geothermal/wind/nuclear-based power generations, etc. We hope that this presentation will convey our interest in teaming up with UM researchers and industry partners to collectively explore future opportunities

Metal Transfer in Arc Welding

A double pulse welding current method is disclosed for the generation and transfer of droplets of welding metal from an electrode wire to a workpiece in an arc welding process. A suitable background direct current level is specified to deliver a desired number of droplets to the weld site. During each cycle of droplet formation and transfer, a first increased current pulse is applied to the electrode and arc to generate a droplet on the tip of and electrode and then a second further increased current pulse is applied to timely separate the droplet from the electrode for transport in the arc to the workpiece. This double-pulse current application reliably produces one droplet per cycle of pulses to deliver a specified number of droplets to the weld site for improved weld quality and reduced spatter or waste of weld metal

Method of Metallurgically Bonding Articles and Article Therefor

An article suitable for metallurgical bonding having a first part having a lower surface, and a second part having an upper surface is disclosed. The lower surface of the first part is disposed at the upper surface of the second part to provide for a faying surface thereat. The faying surface has a plurality of channels having a depth equal to or greater than about 1 micron and equal to or less than about 1000 microns. The article is suitable for metallurgical bonding at the faying surface