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Min Namkung

Bio: Min Namkung is an academic researcher from Langley Research Center. The author has contributed to research in topics: Eddy current & Acoustic emission. The author has an hindex of 13, co-authored 82 publications receiving 583 citations.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors show that exposure of as-produced HiPCo single-walled carbon nanotubes to a camera flash causes ignition, (oxidation) and subsequent coalescence of the Fe catalyst particles, while purified SWNTs do not respond to flashing.
Abstract: The exposure of as-produced HiPCo single-walled carbon nanotubes (SWNTs) to a camera flash causes ignition, (oxidation) and subsequent coalescence of the Fe catalyst particles, while purified SWNTs do not respond to flashing. TEM and electron energy loss spectroscopy (EELS) analysis attribute the phenomena to the pyrophoric oxidation of Fe nanoparticles.

49 citations

Patent
28 Nov 2001
TL;DR: In this article, a giant magnetoresistive flux focusing eddy current device is used to detect deep flaws in thick multilayer conductive materials, which is mounted in a hand-held rotating probe assembly that is connected to a computer.
Abstract: A giant magnetoresistive flux focusing eddy current device effectively detects deep flaws in thick multilayer conductive materials. The probe uses an excitation coil to induce eddy currents in conducting material perpendicularly oriented to the coil's longitudinal axis. A giant magnetoresistive (GMR) sensor, surrounded by the excitation coil, is used to detect generated fields. Between the excitation coil and GMR sensor is a highly permeable flux focusing lens which magnetically separates the GMR sensor and excitation coil and produces high flux density at the outer edge of the GMR sensor. The use of feedback inside the flux focusing lens enables complete cancellation of the leakage fields at the GMR sensor location and biasing of the GMR sensor to a location of high magnetic field sensitivity. In an alternate embodiment, a permanent magnet is positioned adjacent to the GMR sensor to accomplish the biasing. Experimental results have demonstrated identification of flaws up to 1 cm deep in aluminum alloy structures. To detect deep flaws about circular fasteners or inhomogeneities in thick multilayer conductive materials, the device is mounted in a hand-held rotating probe assembly that is connected to a computer for system control, data acquisition, processing and storage.

42 citations

Journal Article
TL;DR: An eddy current probe which provides a null-signal in the presence of unflawed material without the need for any balancing circuitry has been developed at NASA Langley Research Center.
Abstract: An eddy current probe which provides a null-signal in the presence of unflawed material without the need for any balancing circuitry has been developed at NASA Langley Research Center. Such a unique capability of the probe reduces set-up time, eliminates tester configuration errors, and decreases instrumentation requirements. The probe is highly sensitive to surface breaking fatigue cracks, and shows excellent resolution for the measurement of material thickness, including material loss due to corrosion damage. The presence of flaws in the material under test causes an increase in the extremely stable and reproducible output voltage of the probe. The design of the probe and some examples illustrating its flaw detection capabilities are presented.

35 citations

Patent
22 Nov 1994
TL;DR: In this article, a flux-focusing electromagnetic sensor which uses a ferromagnetic flux focusing lens simplifies inspections and increases detectability of fatigue cracks about circular fasteners and other circular inhomogeneities in high conductivity material.
Abstract: A flux-focusing electromagnetic sensor which uses a ferromagnetic flux-focusing lens simplifies inspections and increases detectability of fatigue cracks about circular fasteners and other circular inhomogeneities in high conductivity material. The unique feature of the device is the ferrous shield isolating a high-turn pick-up coil from an excitation coil, The use of the magnetic shield is shown to produce a null voltage output across the receiving coil in the presence of an unflawed sample. A redistribution of the current flow in the sample caused by the presence of flaws, however, eliminates the shielding condition and a large output voltage is produced, yielding a clear unambiguous flaw signal. By rotating the probe in a path around a circular fastener such as a rivet while maintaining a constant distance between the probe and the center of a rivet, the signal due to current flow about the rivet can be held constant. Any further changes in the current distribution, such as due to a fatigue crack at the rivet joint, can be detected as an increase in the output voltage above that due to the flow about the rivet head.

32 citations

Patent
08 May 1996
TL;DR: In this article, a flux-focusing electromagnetic sensor with a ferromagnetic lens was used to detect fatigue cracks and material loss in high conductivity material, and the maximum sensor output was obtained when positioned symmetrically above the crack.
Abstract: A flux-focusing electromagnetic sensor which uses a ferromagnetic flux-focusing lens simplifies inspections and increases detectability of fatigue cracks and material loss in high conductivity material. The unique feature of the device is the ferrous shield isolating a high-turn pick-up coil from an excitation coil. The use of the magnetic shield is shown to produce a null voltage output across the receiving coil in the presence of an unflawed sample. A redistribution of the current flow in the sample caused by the presence of flaws, however, eliminates the shielding condition and a large output voltage is produced, yielding a clear unambiguous flaw signal. The maximum sensor output is obtained when positioned symmetrically above the crack. Hence, by obtaining the position of the maximum sensor output, it is possible to track the fault and locate the area surrounding its tip. The accuracy of tip location is enhanced by two unique features of the sensor; a very high signal-to-noise ratio of the probe's output which results in an extremely smooth signal peak across the fault, and a rapidly decaying sensor output outside a small area surrounding the crack tip which enables the region for searching to be clearly defined. Under low frequency operation, material thinning due to corrosion damage causes an incomplete shielding of the pick-up coil. The low frequency output voltage of the probe is therefore a direct indicator of the thickness of the test sample.

32 citations


Cited by
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Journal ArticleDOI
TL;DR: This Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting and describes the known mechanisms of optical limiting for the different types of materials.
Abstract: The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the functi...

424 citations

Journal ArticleDOI
TL;DR: A review of magnetic non-destructive evaluation methods can be found in this paper, where the authors present a fairly comprehensive summary of works on Barkhausen effect, magnetoacoustic emission, magtenic hysteresis, residual field and magnetically induced velocity change methods.
Abstract: This paper provides a guideline to the literature of magnetic techniques for nondestructive evaluation. Compared with other NDE methods such as ultrasonics or eddy currents, the literature for magnetic methods is relatively small, but one of the difficulties is that it is widely scattered. This review therefore presents a fairly comprehensive summary of works on Barkhausen effect, magnetoacoustic emission, magtenic hysteresis, residual field and magnetically induced velocity change methods that have appeared to date.

275 citations

PatentDOI
TL;DR: The welding of certain polymeric nanofibers can be accomplished by exposure to an intense short burst of light, such as is provided by a camera flash, resulting in an instantaneous melting of the exposed fibers and a welding of the fibers where they are in contact.
Abstract: The welding of certain polymeric nanofibers can be accomplished by exposure to an intense short burst of light, such as is provided by a camera flash, resulting in an instantaneous melting of the exposed fibers and a welding of the fibers where they are in contact. The preferred nanofibers are composed of conjugated, conducting polymers, and derivatives and polymer blends including such materials. Alternatively, the nanofibers can be composed of colored thermoplastic polymeric fibers or opaque polymers by proper selection of the frequency or frequency range and intensity (power) of the light source. The flash welding process can also be used to weld nanofibers which comprise a blend of polymeric materials where at least one of the materials in the blend used to form the nanofiber is a conductive, conjugated polymer or a suitable colored thermoplastic. Alternatively the material blend used to form the nanofibers may comprise a polymeric material containing a colored additive, which is not necessarily a polymer, for example carbon black, or a colored nano-particulate organic or inorganic material, dye or pigment.

213 citations