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This paper focuses on reliability prediction of long-term creep strength for Modified 9Cr-1Mo steel (Gr. 91) which is considered as one of the structural materials of next generation reactor systems. A “Z-parameter” method was introduced to describe the magnitude of standard deviation of creep rupture data to the master curve which can be plotted by log stress vs. The larson-Miller parameter (LMP). Statistical analysis showed that the scattering of the Z-parameter for the Gr. 91 steel well followed normal distribution. Using this normal distribution of the Z-parameter, the various reliability curves for creep strength design, such as stress-time temperature parameter reliability curves (σ-TTP-R curves), stress-rupture time-reliability curves (σ-tr-R curves), and allowable stress-temperature- reliability curves ([σ]-T-R curves) were reasonably drawn, and their results are discussed.

keyword : creep, creep strength, Z-parameter, Larson-Miller parameter, Gr.91 steel

In order to gain better understanding of the selective surface oxidation and its influence on the galvanizability of a transformation-induced plasticity (TRIP) assisted steel containing 1.5 wt.% Si and 1.6 wt.% Mn, a model experiment has been carried out by depositing Si and Mn (each with a nominal thickness of 10 nm) in either monolayers or bilayers on a low-alloy interstitial-free (IF) steel sheet. After intercritical annealing at 800℃ in a N2 ambient with a dew point of -40℃, the surface scale formed on 590 MPa TRIP steel exhibited a microstructure similar to that of the scale formed on the Mn/Si bilayer-coated IF steel, consisting of Mn2SiO4 particles embedded in an amorphous SiO2 film. The present study results indicated that, during the intercritical annealing process of 590 MPa TRIP steel, surface segregation of Si occurs first to form an amorphous SiO2 film, which in turn accelerates the out-diffusion of Mn to form more stable Mn-Si oxide particles on the steel surface. During hot-dip galvanizing, particulate Fe3O4, MnO, and Si-Mn oxides were reduced more readily by Al in a Zn bath than the amorphous SiO2 film. Therefore, in order to improve the galvanizability of 590 TRIP steel, it is most desirable to minimize the surface segregation of Si during the intercritical annealing process.

keyword : metals, annealing, oxidation, scanning electron microscopy, TRIP steel, hot-dip galvanizing

This work investigated the microstructure and mechanical properties of 6063 Al alloy fabricated by shear-drawing (SD) technique where shear and drawing strains were combined together within a predetermined die. To find the optimum condition for sound deformation, three different dies having different inner angle and diameter of the exit channel were prepared. After single deformation of the present sample, the sound deformation took place without an abrupt failure of the sample if the inner angle would be greater than 135˚ in this study, when the channel diameter of the SD die was reduced from 10 to 9 mm. Microstructural observation showed that the inner angle of 135˚ was found to be more effective than that of 150˚ in terms of the alignment of each grain to the shear direction imposed by SD method. In addition, the yield strength of the SD-deformed sample was twice higher than that of the initial counterpart while loosing ductility in tension.

keyword : alloys, drawing, mechanical properties, scanning electron microscopy, SEM

Mg-23.5 wt%Ni-xwt%Cu (x=2.5, 5 and 7.5) samples for hydrogen storage were prepared by melt spinning and crystallization heat treatment from a Mg-23.5 wt%Ni-5 wt%Cu alloy synthesized by the gravity casting method. They were then ground under H2 to obtain a fine powder. Among these samples the Mg- 23.5Ni-2.5Cu sample had the highest hydriding and dehydriding rates after activation. The Mg-23.5Ni-2.5Cu sample absorbed 3.59 and 4.01 wt%H for 10 and 60 min, respectively, at 573K under 12 bar H2. The activated 88(87.5Mg-10Ni-2.5Cu)-5Nb2O5-7NbF5 sample absorbed 2.93 wt%H for 10 min, and 3.14 wt%H for 60 min at 573K under 12 bar H2.

keyword : energy storage materials, hydrogen absorbing materials, mechanical alloying/milling, scanning electron microscopy, SEM

In this study, MOSFET type micro hydrogen gas sensors with platinum catalytic metal gates were designed, fabricated, and their electrical characteristics were analyzed. The devised MOSFET Hydrogen Sensors, called MHS-1 and -2, were designed with a platinum gate for hydrogen gas adsorption, and an additional sensing part for higher gas sensitivity and with a micro heater for operation temperature control. In the electrical characterization of the fabricated Pt-gate MOSFET (MHS-1), the saturated drain current was 3.07 mA at 3.0 V of gate voltage, which value in calculation was most similar to measurement data. The amount of threshold voltage shift and saturated drain current increase to variation of hydrogen gas concentration were calculated and the hydrogen gas sensing properties were anticipated and analyzed.

keyword : hydrogen absorbing materials, sputtering, electrical properties, electrical conductivity/resistivity, MOSFET

100 nm-thick hydrogenated amorphous silicon (α-Si:H) films were deposited on a glass and glass/30 nm Ni substrates by inductively-coupled plasma chemical vapor deposition (ICP-CVD) at temperatures ranging from 100 to 550℃. The sheet resistance, microstructure, phase transformation and surface roughness of the films were characterized using a four-point probe, AFM (atomic force microscope), TEM (transmission electron microscope), AES (Auger electron spectroscopy), HR-XRD(high resolution X-ray diffraction), and micro-Raman spectroscopy. A nano-thick NiSi phase was formed at substrate temperatures >400℃. AFM confirmed that the surface roughness did not change as the substrate temperature increased, but it increased abruptly to 6.6 nm above 400℃ on the glass/30 nm Ni substrates. HR-XRD and micro-Raman spectroscopy showed that all the Si samples were amorphous on the glass substrates, whereas crystalline silicon appeared at 550℃ on the glass/30 nm Ni substrates. These results show that crystalline NiSi and Si can be prepared simultaneously on Ni-inserted substrates.

keyword : Si thin films, ICP-CVD, crystallization, TEM, Low temperature process

The 30 nm-thick Ni layers was deposited on a flexible polyimide substrate with an e-beam evaporation. Subsequently, we deposited a Si layer using a catalytic CVD (Cat-CVD) in a hydride amorphous silicon (α-Si:H) process of Ts=180℃ with varying thicknesses of 55, 75, 145, and 220 nm. The sheet resistance, phase, degree of the crystallization, microstructure, composition, and surface roughness were measured by a four-point probe, HRXRD, micro-Raman spectroscopy, FE-SEM, TEM, AES, and SPM. We confirmed that our newly proposed Cat-CVD process simultaneously formed both NiSi and crystallized Si without additional annealing. The NiSi showed low sheet resistance of <13Ω/□, while carbon (C) diffused from the substrate led the resistance fluctuation with silicon deposition thickness. HRXRD and micro-Raman analysis also supported the existence of NiSi and crystallized (>66%) Si layers. TEM analysis showed uniform NiSi and silicon layers, and the thickness of the NiSi increased as Si deposition time increased. Based on the AES depth profiling, we confirmed that the carbon from the polyimide substrate diffused into the NiSi and Si layers during the Cat-CVD, which caused a pile-up of C at the interface. This carbon diffusion might lessen NiSi formation and increase the resistance of the NiSi.

keyword : nickel silicide, Cat-CVD, hydrogenated amorphous silicon, polycrystalline silicon, polyimide films

To develop wide-band noise absorbers with a special design for low-frequency performance, this study proposes a tin oxide (Sn-O) thin films as the noise absorbing materials in a microstrip line. Sn-O thin films were deposited on polyimide film substrates by reactive sputtering of the Sn target under flowing O2 gas, exhibiting a wide variation of surface resistance (in the range of 100-105Ω) depending on the oxygen partial pressure during deposition. The microstrip line with characteristic impedance of 50Ω was used for the measurement of noise absorption by the Sn-O films. The reflection parameter (S11) increased with a decrease of surface resistance due to an impedance mismatch at the boundary between the film and the microstrip line. Meanwhile, the transmission parameter (S21) diminished with a decrease of surface resistance resulting from an Ohmic loss of the Sn-O films. The maximum noise absorption predicted at an optimum surface resistance of the Sn-O films was about 150Ω. For this film, greater power absorption is predicted in the lower frequency region (about 70% at 1 GHz) than in conventional magnetic sheets of high magnetic loss, indicating that Ohmic loss is the predominant loss parameter for the conduction noise absorption in the low frequency band.

keyword : thin films, vapor deposition, electrical properties, electrical conductivity/resistivity, noise absorbers

The mechanism behind super-filling of high-aspect-ratio features with Cu by catalyst-enhanced chemical vapor deposition (CECVD) coupled with plasma treatment is described and the metrology required to predict the filling feasibility is identified and quantified. The reaction probability of a Cu precursor was determined as a function of substrate temperature. Iodine adatoms are deactivated by the bombardment of energetic particles and also by the overdeposition of sputtered Cu atoms during the plasma treatment. The degree of deactivation of adsorbed iodine was experimentally quantified. The quantified factors, reaction probability and degree of deactivation of iodine were introduced to the simulation for the prediction of the trench filling aspect by CECVD coupled with plasma treatment. Simulated results show excellent agreement with the experimental filling aspects.

keyword : electrical/electronic materials, vapor deposition, surface, computer simulation, Cu CECVD

We fabricated the silicon nanodots using the low pressure chemical vapor deposition technique to investigate their electron field emission characteristics. Atomic force microscope measurements performed for the silicon nanodot samples having various process parameters, such as, deposition time and deposition pressure, revealed that the silicon nanodots with an average size of 20 nm, height of 5 nm, and density of 1.3×10(11) cm-2 were easily formed. Electron field emission measurements were performed with the silicon nanodot layer as the cathode electrode. The current-voltage curves revealed that the threshold electric field was as low as 8.3 V/㎛and the field enhancement factor reached as large as 698, which is compatible with the silicon cathode tips fabricated by other techniques. These electron field emission results point to the possibility of using a silicon-based light source for display devices.

keyword : nanostructured materials, vapor deposition, electrical properties, AFM, silicon nanodots, electron field emission