Si-Kyung Choi

Effect of nitrogen on high temperature low cycle fatigue behaviors in type 316L stainless steel

Abstract Strain-controlled low cycle fatigue (LCF) tests were conducted in the temperature range of RT–600°C and air atmosphere to investigate the nitrogen effect on LCF behavior of type 316L stainless steels with different nitrogen contents (0.04–0.15%). The waveform of LCF was a symmetrical triangle with a strain amplitude of ±0.5% and a constant strain rate of 2×10 −3 /s was employed for most tests. Cyclic stress response of the alloys exhibited a gradual cyclic softening at RT, but a cyclic hardening at an early stage of fatigue life at 300–600°C. The hardening at high temperature was attributed to dynamic strain aging (DSA). Nitrogen addition decreased hardening magnitude (maximum cyclic stress — first cyclic stress) because nitrogen retarded DSA for these conditions. The dislocation structures were changed from cell to planar structure with increasing temperature and nitrogen addition by DSA and short range order (SRO). Fatigue life was a maximum at 0.1% nitrogen content, which was attributed to the balance between DSA and SRO.

A study on the crystallographic orientation with residual stress and electrical property of Al films deposited by sputtering

Abstract Al metallization on the Si substrate was carried out by rf magnetron sputtering as a function of working pressure and the negative bias voltage. The residual stress, the resistivity, and the crystallographic orientation were investigated. The residual stress of Al thin films fabricated in the range of 1 to 40 mTorr showed tensile stress from ∼125 MPa to 0. Applying negative bias of −200 V to the substrate at a fixed working pressure of 1 mTorr, the residual stress changed to the compressive stress of ∼70 MPa. The crystallographic orientation showed a random orientation at zero stress state. As the compressive stress increased, it changed to (200) preferred orientation. This relationship was explained by the strain energy. But it changed to (111) preferred orientation as the tensile stress increased. Resistivity in the Al thin films increased as the compressive stress increased. Ar entrapment by the bombardment effect was responsible for this relationship.

Effect of nitrogen on the dynamic strain ageing behaviour of type 316L stainless steel

The dynamic strain ageing (DSA) behaviours of type 316L stainless steels containing different nitrogen contents (0.01–0.15 wt% N) were studied in tension under varying strain rates (1 × 10−2−2 × 10−4s−1) and the test temperatures (R.T.–1023 K). The temperature range for DSA was moved to higher temperature for increasing nitrogen contents. The critical strain, εc for the onset of serration increased with nitrogen content at 773 K and then became almost constant at 873 K. Type A and B serrations were observed at 873 K with the value of the strain required to effect the transition from type A to type B serration increasing for nitrogen contents upto 0.1 wt% and then becoming saturated. The activation energy for DSA was 23.4–26.2 kcal mol−1 (97.8–109.5 kJ mol−1) at the onset and 65.0–76.6 kcal mol−1 (271–320.2 kJ mol−1) at the end of serration. The lower activation energy was related to vacancy diffusion and the higher activation energy was attributed to the diffusion of chromium to dislocations. The activation energy for DSA was slightly increased with nitrogen addition. DSA was retarded by an increase in the nitrogen content since nitrogen reduced the chromium diffusion to dislocations due to a strong interaction between the nitrogen and chromium.

Surface characterization of diamond films polished by thermomechanical polishing method

Abstract The rough grown surface of a diamond film deposited by the hot filament chemical vapour deposition (CVD) method was polished by thermomechanical polishing on a hot iron plate at 1173 K. The surface composition and structure of the polished diamond film were analysed by Raman spectroscopy, Auger electron spectroscopy (AES), thin film X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). The graphitization of the near-surface region of the diamond film by the iron plate was observed during thermomechanical polishing. At the diamond film/Si interface, β-SiC was observed during the early stages of polishing, but after prolonged treatment the graphitization of the diamond film occurred rapidly.

Facile external treatment for efficient nanoscale morphology control of polymer solar cells using a gas-assisted spray method

A facile and effective treatment method for controlling the morphology of bulk heterojunction (BHJ) structured polymer-based solar cells (PSCs) using a gas-assisted spray (g-spray) technique was demonstrated. High-efficiency BHJ-PSCs were fabricated using a g-spray method that can be adapted to large-scale high-throughput continuous production, and the bulk film morphology and internal nanomorphology of the active layers were well manipulated using a sprayed solvent overlayer (SSO) treatment. The efficient nanomorphology evolution, which is a prerequisite for obtaining high performance BHJ-PSCs, was confirmed by X-ray diffraction, UV-Vis, photoluminescence, and transmission electron microscopy analysis. The SSO treatment was a simple and rapid process that could be carried out at room temperature, unlike conventional external treatment (ET) methods such as solvent- or thermal-assisted treatment, which typically require a prolonged time (>1 h) or relatively high temperature (>110 °C). After SSO treatment, the PSC performance was enhanced remarkably. The power conversion efficiency (PCE) of the g-sprayed PSCs after SSO treatment was 2.99%, which is higher than that of a solvent vapor treated device (2.42%) and thermally annealed devices (2.61%). Further optimization of the nanomorphology was achieved by sequentially developing P3HT and PCBM. By combining thermal annealing with the SSO treatment, the P3HT/PCBM interfacial area could be enhanced; this enhancement was induced by the PCBM diffusion into the space among pre-assembled P3HT nanofibrils, which in turn promoted their bi-continuity. This means of sequential nanomorphology development further enhanced the PCE (3.35%), which was higher than the other reported values for PSCs using spray methods. Considering that the SSO treatment is a facile room temperature process that requires a short time, these results suggest that the g-spray method can be successfully applied to the continuous production of PSCs.

The influence of an extrinsic interfacial layer on the polarization of sputtered BaTiO3 film

As an origin of degradation of remnant polarization in Pt∕BaTiO3∕Pt capacitor structure, an interfacial layer formed at the interface of BaTiO3 film and a Pt bottom electrode is considered. BaTiO3 films were deposited on two types of bottom electrodes (La0.5Sr0.5CoO3 and Pt) by the radio frequency magnetron sputtering method and both capacitors showed a microstructural similarity with strong preferred orientations. However, a Pt∕BaTiO3∕La0.5Sr0.5CoO3 capacitor exhibited a saturated hysteresis loop with the remnant polarization (2Pr) of 6μC∕cm2, and for the Pt∕BaTiO3∕Pt structure, the polarization-voltage curve revealed a linear dielectric characteristic. From a cross-sectional high-resolution transmission electron microscope analysis of the Pt∕BaTiO3∕Pt capacitor showing the linear dielectric property, an interfacial layer with an amorphous structure as well as a multidomain structure in the interior of the BaTiO3 film were observed. It is concluded that the interfacial layer might help degradation of pol...

Influence of film density on residual stress and resistivity for Cu thin films deposited by bias sputtering

Abstract Cu thin films were deposited by bias sputtering and the film density was measured by grazing incidence X-ray reflectivity. The influence of the experimentally measured film density on residual stress and resistivity, which previously few studies reported on, was investigated. Without bias voltage, the relative film density was low and the tensile stress was high. With increasing bias voltage, the tensile stress decreased and saturated to nearly zero at a bias voltage of −100 V, while the film density increased and saturated to nearly bulk value at a bias voltage of −100 V. These results were consistent with those from previously reported molecular dynamic simulations. From the consideration of the Morse potential for relatively dense films, it was possible that tensile stress decreased as the film density increased. The resistivity is high in Cu thin films deposited without bias voltage. With increasing bias voltage, the resistivity decreased and saturated to nearly bulk value at a bias voltage of −100 V. The resistivity is thought to be influenced by the film density.

Domain structure in a micron-sized PbZr1−xTixO3 single crystal on a Ti substrate fabricated by hydrothermal synthesis

Hydrothermal synthesis is an attractive method for the deposition of ferroelectric thin films because it can realize a low processing temperature of 200 °C or less. In this study, micron-sized PbZr1−xTixO3 (PZT) single crystals and thin films were fabricated on a Ti substrate at 160 °C by hydrothermal synthesis. The ferroelectricity of the crystals was investigated using local piezoresponse hysteresis, while their domain structure was investigated using high-resolution transmission electron microscopy (HRTEM) measurement. The isolated PZT single crystal exhibited hysteresis behavior of the piezoresponse, which represents ferroelectricity. From the HRTEM observations of a crystallite in polycrystalline PZT thin film, we found a twinned domain structure in the form of a … c/a/c/a … polydomain pattern. On the basis of the HRTEM observations, we proposed a three-dimensional domain structure for the isolated PZT single crystal; the c/a domain walls form along coherent {101} planes in the tetragonal cell. This ...

Switchable single c-domain formation in a heteroepitaxial PbTiO3 thin film on a (001) Nb-SrTiO3 substrate fabricated by means of hydrothermal epitaxy

We used hydrothermal epitaxy to fabricate a heteroepitaxial PbTiO3 (PTO) thin film on a Nb-doped (001) cubic SrTiO3 (NSTO) substrate while avoiding the phase transition. By means of transmission electron microscopy and piezoresponse force microscopy analyses, it was confirmed that no a domain formed in the heteroepitaxial PTO film; the film had a single +c-domain structure at an as-synthesized state. From the measurement of the polarization-voltage hysteresis curve, large remanent polarization (2Pr:144μC∕cm2) resulting from the single +c-domain structure and an imprint induced by an asymmetric electrode configuration were observed. It is suggested that the single +c-domain structure was caused by the interface of the PTO film and solution rather than the interface of the PTO film and NSTO electrode under the hydrothermal conditions. This hydrothermally synthesized PTO film is anticipated to be very suitable for high-performance engineering applications.

Effect of the thickness of plasma-sprayed coating on bond strength and thermal fatigue characteristics

NiCrAlY bond coat and ZrO2–8 wt% Y2O3 top coat with various thicknesses were deposited on Hastelloy X by plasma spraying. Residual stress was calculated by the finite element method (FEM) to explain the variations in the bond strength and thermal fatigue characteristics with the thickness of the bond coat and top coat. The bond strength of thermal barrier coatings (TBCs) increased with decreasing maximum residual stress in the y-direction of the top coat. The thermal fatigue characteristics increased with decrease of the maximum principal residual stress of the top coat and the thickness of oxidation layer of the bond coat.