Hwa Shik Youn

Current Status of the Synchrotron Small-Angle X-ray Scattering Station BL4C1 at the Pohang Accelerator Laboratory

The small-angle X-ray scattering (SAXS) beamline BL4C1 at the 2.5 GeV storage ring of the Pohang Accelerator Laboratory (PAL) has been in its first year of operation since August 2000. During this first stage it could meet the basic requirements of the rapidly growing domestic SAXS user community, which has been carrying out measurements mainly on various polymer systems. The X-ray source is a bending magnet which produces white radiation with a critical energy of 5.5 keV. A synthetic double multilayer monochromator selects quasi-monochromatic radiation with a bandwidth of ca. 1.5%. This relatively low degree of monochromatization is sufficient for most SAXS measurements and allows a considerably higher flux at the sample as compared to monochromators using single crystals. Higher harmonics from the monochromator are rejected by reflection from a flat mirror, and a slit system is installed for collimation. A charge-coupled device (CCD) system, two one-dimensional photodiode arrays (PDA) and imaging plates (IP) are available as detectors. The overall performance of the beamline optics and of the detector systems has been checked using various standard samples. While the CCD and PDA detectors are well-suited for diffraction measurements, they give unsatisfactory data from weakly scattering samples, due to their high intrinsic noise. By using the IP system smooth scattering curves could be obtained in a wide dynamic range. In the second stage, starting from August 2001, the beamline will be upgraded with additional slits, focusing optics and gas-filled proportional detectors.

Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

Abstract Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l 1 (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness d c , not the amorphous layer thickness d a . The observed d c values are found to be always smaller than d a , regardless of polymer composition. d c is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike d c , d a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d a distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.

X-ray imaging of various biological samples using a phase-contrast hard X-ray microscope

In this study, we visualized the internal structures of various bio‐samples and found the optimum conditions of test samples for the 7 keV hard X‐ray microscope of the Pohang light source. From the captured X‐ray images, we could observe the intercellular and intracellular structures of dehydrated human cells and mouse tumor tissues without using any staining materials in a spatial resolution better than 100 nm. The metastasized lung tissue, which was several tens of micrometers in thickness, was found to be very well suited to this hard X‐ray microscope system, because it is nearly impossible to observe such a nontransparent and thick sample with a high spatial resolution better than 100 nm using any microscopes such as a soft X‐ray microscope, an optical microscope, or an electron microscope. Microsc. Res. Tech., 2008.

Electric-field-induced structural modulation of epitaxial BiFeO3 multiferroic thin films as studied using x-ray microdiffraction

An in situ method, called synchrotron x-ray microdiffraction, was introduced to examine the electric-field-induced structural modulation of the epitaxially grown pseudotetragonal BiFeO3 thin film. To evaluate the d spacing (d001) from the measured intensity contour in the 2θ-χ space, the peak position in each diffraction profile was determined by applying two-dimensional Lorentzian fitting. By tracing the change of d spacing as a function of the applied electric field and by examining the Landau free energy function for P4mm symmetry, the authors were able to estimate the two important parameters that characterize the field-induced structural modulation. The estimated linear piezoelectric coefficient (d33) at zero-field limit is 15pm∕V, and the effective nonlinear electrostrictive coefficient (Qeff) is as low as ∼8.0×10−3m4∕C2.

Hard x-ray microscopy with a 130nm spatial resolution

We have developed a new full field x-ray microscope at the beam line 1B2 of PLS (Pohang Light Source). It was designed to operate at the energy between 7 and 12keV. High efficiency hard x-ray zone plates, condenser and objective, have been chosen as x-ray lenses. The spatial resolution of the microscope is better than 130nm at 6.95keV.

Observations of a human hair shaft with an x-ray microscope.

We observed the internal structures of a human hair shaft using x-ray microscopes with a spatial resolution in the range from a few microns to less than 100 nm. The energy of the x-ray used is 6.95 keV. The Zernike phase contrast together with a spatial resolution better than 100 nm enabled us to see the cuticles of scales, the cortex of macrofibrils and the medulla. All these internal features and more can easily be observed with no sample preparation including staining.

Local strain-induced 90° domain switching in a barium titanate single crystal

Local strain analysis for a BaTiO3 single crystal under electric loads was conducted using x-ray microdiffraction technique. During the polarization reversal, local 90° domain switchings were observed at certain positions instead of uniform 180° domain switching. Field-dependent strain analysis demonstrated that inhomogeneous domain switching was strongly related to local strain distributions. The results can be understood in terms of competition between elastic and electrostatic energies and provide a crystallographic evidence for the role of local strains on 90° domain switching.

USE OF PHOTOELECTRON MICROSCOPES AS X-RAY DETECTORS FOR IMAGING AND OTHER APPLICATIONS

We demonstrate with practical tests that a photoelectron emission microscope (PEEM) can be advantageously used as a high-lateral-resolution detector of X-rays. The advantages of this approach are discussed, in particular for coherence-based techniques

Supramolecular assembly of collagen fibrils into collagen fiber in fish scales of red seabream, Pagrus major.

Supramolecular assembly of collagen fibrils into collagen fiber and its distribution in fish scales of red seabream, Pagrus major, were investigated. By virtue of Zernike phase-contrast hard X-ray microscopy, it has been firstly observed that collagen fiber consists of helical substructures of collagen fibrils wrapped with incrustation. As it close to the scalar focus (that is, with aging), loosened- and deteriorated-helical assemblies started to be observed with loosing wrapping incrustation, indicative of the distortion of the basic helical assembly. Various distributions and packing arrangements of collagen fibers were observed dependent on subdivisions of fish scale. Freshly growing edge region of fish scale, embedded into fish skin, showed rarely patched and one directionally arranged collagen fibers, in which specifically triple helical assemblies of collagen fibrils were found. On the contrary, relatively aged region of the rostral field close to the scalar focus displayed randomly directed and densely packed collagen fibers, in which loosened- and deteriorated-helical assemblies of collagen fibrils were mostly found. Our results have demonstrated that hard X-ray microscope can be a powerful tool to study in situ internal structure of biological specimens in an atmospheric pressure.

Properties of superconducting MgB2 single crystal grown by a modified flux method

We report on the growth of MgB2 single crystals with a large thickness of about 100 μm and a clear hexagonal prismatic morphology. On the low-field magnetization curve M(T), a superconducting transition was observed at 39 K. Using the x-ray diffractometer equipped with a microprobe, we obtained Laue patterns of MgB2 single crystal from a piece of the grown crystal and investigated the impurities introduced during the growing process using Auger electron spectroscopy.