Sekyung Lee

Facile fabrication of 2-dimensional arrays of sub-10 nm single crystalline Si nanopillars using nanoparticle masks.

A simple procedure for the fabrication of sub-10 nm scale Si nanopillars in a 2-D array using reactive ion etching with 8 nm Co nanoparticles as etch masks is demonstrated. The obtained Si nanopillars are single crystalline tapered pillar structures of 5 nm (top) x 8 nm (bottom) with a density of approximately 4 x 10(10) pillars cm(-2) on the substrate, similar to the density of Co nanoparticles distributed before the ion etching process. The uniform spatial distribution of the Si nanopillars can also be patterned into desired positions. Our fabrication method is straightforward and requires mild process conditions, which can be extended to patterned 2-D arrays of various Si nanostructures.

Eddy current test of steel tube employing electromagnet technique for dc magnetization

The impedance of a test coil was measured as a function of the dc current of an electromagnet, and the determination method of magnetization level with the normalized impedance diagram is discussed. An eddy current test for the outer side of a steel tube wall was also performed. Test results show that the impedance change of the test coil is brought about by the eddy current perturbation and the change of magnetization level. These effects on the flaw inspection are also analyzed.

Time-domain reflection field analysis for ultrasonic evaluation of thin layered media

Abstract In conventional ultrasonic testing of a thin layer with thickness less than about twice the wavelength, successive echo signals become inseparable. Recently, a new time-domain analysis technique to determine the thickness or the wave velocity of a thin plate was developed and through-transmission experiments were reported. We have studied the validity of the technique and its applicability to contact testing under a reflection field system. We found a mistake in the derivation of the reconstructed reflection field equation reported, and a correction has been made. We performed the experiment in a reflection field system using contact type transducers of centre frequency 1, 2.25, and 5 MHz with water layers of thickness ranging from 0.06 to 1.44 mm (0.04 ≤ thickness/wavelength h ∗ λ ) larger than about 0.4, 3% for h ∗ λ larger than 0.8, and 1% for h ∗ λ larger than 2. The time domain technique with our corrected equation was applicable to the evaluation of a thin layered medium under reflection system using contact type transducers.

Ab initio study on fragmentation of C60 into C58 and C2

Abstract First-principles pseudopotential calculations were carried out to evaluate barrier heights during the process of fragmentation of the perfect C60 into the perfect C58 and the C2. Three important paths were taken into account: (i) the perfect C60 → a defect C58 + C2 → the perfect C58 + C2 (removal of single-bonded two carbon atoms), (ii) the perfect C60 → a defect C58 + C2 → a defect C58 + C2 → the perfect C58 + C2 (removal of double-bonded two carbon atoms), and (iii) the perfect C60 → a defect C60 → the perfect C58 + C2. The third process is energetically favored.

Development and application of ultrasonic transducer for intensive focusing of Rayleigh waves

Abstract Intensive focusing of Rayleigh waves was realized by the fabrication of a curved PZT (Pb(Zr 2 Ti)O 3 ) piezoelectric element having a new shape. Two- and three-dimensional beam profiles of the focusing transducer employing the curved PZT element were obtained by a contact scan on a steel block with flat bottom holes. The beam intensity was higher, about 35 dB, at the focal distance and the signal-to-noise ratio was higher, about 17 dB, than the focusing transducer employing a flexible PVDF (polyvinylidene fluoride) film in the earlier study. The −6 dB and −3 dB lateral widths at the focal point were 0.61 mm and 0.46mm, and the −6 dB and −3 dB axial widths were about 5.5mm and 4.0mm, respectively. The axial stress field showed a good agreement with the overall pattern estimated by a simplified model. The ratio of the detected smallest flaw size to the wavelength of Rayleigh waves for the center frequency was about 0.29. As an application, a single time scan was performed along the flat bottom holes (FBHs) at the focal distance, and an image to show sharp peaks with amplitude depending on the size of the FBHs was obtained. The focusing transducer showed the characteristics of a narrow lateral resolution, high energy, and high signal-to-noise ratio, and was practical for the detection of surface flaws smaller than the wavelength.

Atomic Structure of Si(100) Surfaces

The structure of a clean Si(100) and a Ni-contaminated si(100) was investigated using scanning tunneling microscopy. The clean Si (100) shows the 2 × 1 reconstruction with a surface dimer vacancy density less than 2%. The major defects on the clean surface are a single dimer vacancy and the C defect. A small amount of Ni on the surface drastically changes the surface structure and produces 2 × n reconstructions. The formation of vacancy clusters is favored. A rebonded SB step is preferred on the clean Si(100) while a nonrebonded SB step with a split-off dimer is mainly observed on the Ni-contaminated Si(100) and in the vicinity of dimer vacancies of the lower terrace on the clean Si(100).

Magnetic effects in eddy current nondestructive method

Magnetic effects in eddy current nondestructive method are studied employing an extension of the general ideas of Globus for the magnetization processes of polycrystalline materials. The reversible permeability depends on the residual magnetization of the material. The relation between these two quantities is derived from the Globus model. An expression is derived for the impedance of a single‐turn coil above a ferromagnetic conducting half‐space. From this expression, flaw effects and liftoff effects are analyzed, and theoretical treatment for the evaluation of residual magnetization is described.

Direct and continuous measurement of depth-dependent reflection coefficients of Rayleigh waves for surface discontinuity: Normal slot

A new method and principle were presented for the continuous measurement of the depth-dependent reflection coefficient of Rayleigh waves or their pulse interacting with various surface discontinuities. The method was to record the reflection echo amplitudes for varying depth during scanning the Rayleigh wave pulse with a small lateral beam width along a surface discontinuity with a small slope in depth. In the experiments using a declined slot and intensively focused Rayleigh waves, the coefficients were continuously measured with quite good reproducibility in the range of 0.05<d/λ (slot-depth/Rayleigh wave wavelength) <2.57. In 0.05<d/λ<1.4, the results showed good agreement with other experimental results that had been determined by conventional method [L. J. Bond, Ultrasonics 71, 71–77 (1979); M. Hirao et al., J. Acoust. Soc. Am. 72, 602–606 (1982); H. M. Frost et al., Proc. Ultrason. Symp. IEEE (1975), 604–607] as intermittent point-to-point data and several numerical results [L. J. Bond, Ultrasonics ...

Eddy Current Test of Fin Tubes for a Heat Exchanger.

ABSTRACT Eddy current probes were designed to be suitable for the test of fin tubes. Fin tubes, often used for heat exchangers, have uneven outer and inner surfaces to enhance the heat emission. The surface roughness makes it difficult to detect flaws employing eddy current test (ECT). In order to overcome the difficulties we performed two kinds of works, one is the development of ECT probes, and the other is the signal processing including fast Fourier transform and digital filtering. In the development of ECT probes, we adopted empirical design method. Our ECT probes for the fin tubes are inside type. And we are specially concerned about geometric features such as the widths of the coils composing an ECT probe. We fabricated four probes with various coil widths. Eddy current test was performed using those ECT probes on artificially flawed specimens. After analyzing the output signals, we found that, for the effective testing, the width of a coil should be determined considering the pitch of the fins of a tube. And we also learned that the frequency filtering could improve the s/n.

Atomic structure and energetics of vacancies in a sublayer of GaAs(110)

The energetics and structure of vacancies in the second layer of GaAs(110) were examined using the ab initio total energy calculation method. Structural changes due to the presence of the vacancies are substantial. The changes are different in shape, depending on the type of vacancy and charged states. Both the Ga vacancy and the As vacancy in the second layer of the surface favor negatively charged states.