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The presence of a second phase in the Mg–8Al–4Ca (at. %) alloy plays a significant role on both its corrosion behavior and the chemical conversion coating processes. Using scanning Kelvin probe force microscopy (SKPFM), a lower Volta-potential of the second phase present on the surface has been measured. The β-Al-Ca phase has a higher electrochemical activity than the α-Mg matrix and may act as the micro-galvanic anode in a local electrochemical corrosion process. Transmission electron microscopy (TEM) examinations reveal that the β-Al-Ca phase is more susceptible to corrosion than the α-Mg matrix in an aqueous solution, and its higher activity and higher corrosion rate accelerate the hydrogen evolution rate on the α-Mg matrix in the cerium (Ce) conversion coating process. It's also been discovered that by immersing the bare Mg–Al–Ca alloy in deionized (DI) water, the β-Al-Ca phase, exposed on the surface, can be dissolved and converted in situ into aluminum hydroxide (Al(OH)3), and the Ce conversion coating can be deposited via replacement reactions in the subsequent conversion coating process. A thicker Ce coating with smaller blisters has then been produced on the DI-treated Ce-coated Mg specimen; it indeed improves the corrosion resistance.

ABSTRACTThe purpose of this study was to examine the relationships among homework time, homework frequency, and learning achievements of Taiwanese students. Applying a 2-level hierarchical linear modeling on the TIMSS 2007 and 2011 data, our findings were as follows: (1) Within the context of TIMSS 2007 and 2011 students, the frequency of mathematics homework positively predicted mathematics learning achievements; (2) Within the context of TIMSS 2007 students, the amount of time spent doing mathematics homework positively predicted mathematics learning achievements. (3) Among TIMSS 2011 students, homework time in mathematics of 1 to 15 minutes, 16 to 30 minutes, 31 to 60 minutes, and 61 to 90 minutes all could positively predict mathematics learning achievements of Taiwanese 8th-grade students, except for homework time more than 90 minutes. It revealed that in relation to mathematics homework time, too much homework might be as bad as not enough.

The influence of grain refinement by high pressure torsion (HPT) on microstructure of the Mg-1%Ca alloy was investigated by scanning and transmission electron microscopy. To The microhardness measurements and tensile tests of small samples were used to determine the mechanical properties of UFG samples. It was found that a uniform structure with an average grain size of 210 nm processed by HPT lead to high microhardness equal to 100 HV. Investigations of thermal stability demonstrated that additional heat treatment of the UFG samples at 250 oC provides good combination of high strength of 245 MPa and ductility of 4%.

ABSTRACTSeveral advanced nickel-chromium-molybdenum high strength lowalloy steels newly developed by our research team exhibit excellent mechanical strength, toughness and hardenability. However, the phenomenon of hydrogen-induced embrittlement will easily occur for these high strength steels. In this research, the hydrogeninduced embrittlement of 8625-Modified steel (8625M steel) was studied. Experimental results show that the dominant hydrogen trapping site of the 8625M steel is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogens trapped in the dislocations are diffusible. The as-quenched 8625M steel has the highest dislocation density and accordingly the highesthydrogen content after hydrogen charging. This makes the asquenched 8625M steel exhibit severe hydrogen embrittlement. After tempering at 200°C and 300°C, the dislocation density drops, and hence these tempered specimens have lower ultimate tensile strength loss. After 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again, being ascribed to the formation of needlelike and film-like cementite which will weaken the strength of martensite. After 500°C tempering, the 8625M steel has the lowest dislocation density, and the inter-lath cementite become discontinuous and spheroidal, making the 500°C tempered specimen have the lowest ultimate tensile strength loss and the highest elongation after hydrogen charging in this study.

The demand for new materials that provide excellent structural performance while reducing weight and being cost-effectively manufactured is increasing. For applications with high strength requirements, ultra-high strength steels (UHSS) have been widely used. However, with such a high strength level, UHSS are very sensitive to the hydrogen that could be ease by the tempering process. In this research, the correlation of hydrogen and tempering process on commercial UHSS 15B30 has been studied. Results show that the tensile strength (TS) of as-quenched 15B30 is about 1900MPa. After tempering treatment of the quenched 15B30, the TS decreases from 1600MPa to 1200MPa with tempering temperature increased from 200°C to 400°C. The 15B30 specimens, being subjected to hydrogen charge, exhibit the dramatic reduction of mechanical strengths.

Back-end defect can degrade the production yield and cause a significant quality problem in direct extrusion of the aluminum alloys. This study is concerned with two types of back-end defects, i.e., the funnel defect and the internal oxide ring, which are usually found in the extrudates of aluminum alloys. Hot top and beveled edge dummy blocks are designed to prevent these defects. Due to the high costs of the experimental studies, the effect of two designed dummy blocks on the flow pattern of a deformed billet of aluminum alloys is analyzed by finite element method (FEM) Deform 3D package software. Using simulation, a hot top dummy block with a concave-center is designed to eliminate the formation of the funnel defect. A hot top capacity C is defined at the fixed depth of the concave: C = 1 is a suitable criterion for the design of the hot top dummy block. A beveled edge dummy block is designed to affect and to impede the back flow of the surface layer of the billet into extrudates of aluminum alloys.

In this study, a novel yet general strategy is proposed and demonstrated to enhance the energy storage density (ESD) of dielectric capacitors by introducing a built-in electric field in the dielectric layer, which increases the applied electric field required to polarize the dielectric. By using the top and bottom electrodes of different work functions, a built-in electric field is created in the antiferroelectric layer, which leads to an increase in the critical fields for the antiferroelectric-ferroelectric phase transition and thus the enhancement of ESD. Accordingly, the TiO2/ZrO2/TiO2antiferroelectric capacitor with asymmetric electrodes demonstrates an ultra-high ESD of 114.5 J cm−3, which is improved by ~21% compared to the antiferroelectric capacitor without a built-in field, along with high efficiency of 76%. The achieved ESD is record-high among the HfO2/ZrO2-based lead-free antiferroelectric thin-film capacitors. The result indicates that engineering the built-in electric field can be an effective and promising approach to increasing the ESD for electrostatic supercapacitors.

A new approach to extracting periodicity of a regular texture based on autocorrelation functions is proposed in this paper. From the peaks in the autocorrelation function of an image, the displacement vectors and texture primitives in the image can be effectively determined. Moreover, the area of the extracted texture primitive is relatively small. This fact benefits in further processing of the image, such as data compression, feature extraction, and pattern analysis. The proposed approach needs no preprocessing, such as image thresholding or quantization, on the input image. However, it keeps tolerant to distortions in the texture, such as noises, intensity changes, and geometric variants. A synthesized texture with high regularity can be constructed by arranging the obtained texture primitives according to the displacement vectors. Experimental results support the effectiveness and efficiency of the proposed approach.

The conductivity of composite solid polymer electrolyte (SPE) made of different compositions of poly(ethylene oxide) (PEO), LiClO4and fiber was investigated in this study. Results obtained through alternating current (AC) impedance measurements demonstrated that the conductivity of the SPE was much improved by blending fiber into it. Moreover, increasing the composition of fiber added leads, thereby increasing the conductivity of the composite SPE. The average conductivity of the composite SPE was 10−4S/cm at 25 °C. Performance in thermal properties was also investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments. Although the mechanical strength of the composite SPE was not better than that of other materials as expected, the fiber added made it more stable.

Two algorithms for polygonal approximation of a two-dimensional (2D) shape boundary are proposed in this paper. If the number of vertices in the obtained polygonal representation is large, the representation will characterize the shape boundary with high accuracy, while the cost for storing or transmitting the representation will be high. That is, the larger the compression ratio is, the more detail is lost in the obtained polygonal representation. The proposed first algorithm automatically determines a suitable spread parameter of a Gaussian filter for smoothing a shape boundary. No human intervention is required in this algorithm. Curvature extrema in the smoothed boundary are used as vertices of the polygonal representation. The second algorithm allows users to specify a lower bound of the compression ratio in the obtained polygonal representation. This algorithm produces a polygonal representation of the given shape boundary, whose compression ratio is only a little larger than or equal to the given lower bound. The proposed algorithms are computationally simple and efficient. Experimental results show the proposed algorithms are indeed efficient and effective; they are very useful in early processing of object recognition and analysis.

Polycrystalline LiMn2O4thin films are deposited by radio frequency (rf) magnetron sputtering followed by air annealing at 600 and 400°C. The films are then treated with an rf driven oxygen plasma. The crystallographic structures and surface morphologies of the LiMn2O4thin films are studied. Plasma treatments induce different trends of microstructural evolution in 600 and 400°Cannealed films. The microstructural evolutions are induced by the combination effects of plasma treatments and air annealing. The results can be interpreted by the atomic relaxation and resputtering models. For both high and low temperature annealed films, a well-tuned plasma treatment can result in films with a dense subsurface microstructures and a grain size of ∼10nm, which shows a greatly enhanced electrochemical performances. Under optimal treatments conditions, the electrochemical performances of the LiMn2O4films can be greatly improved. The treated samples are tested under harsh conditions, including deep discharge to 1.5Vand cycling at elevated temperature of 60°C, and such subsurface structure is found to suppress the development of Li1+xMn2O4rock-salt phase.

Polycrystalline LiMn2O4thin films were deposited by radio-frequency (rf) magnetron sputtering followed by annealing at 600°Cin air. The films were then treated with an rf-driven oxygen plasma. The crystallization and surface morphology of LiMn2O4thin films were correlated with rf powers. The treated samples were tested under harsh conditions such as deep discharge to 1.5 V and cycling at an elevated temperature of 60°Cto verify the electrochemical performances of the LiMn2O4cathodes. The oxygen plasma treatments significantly improved the electrochemical properties of LiMn2O4thin films. The results showed that the plasma treatments not only resulted in surface densification but also induced finer grains in the subsurface region (10–100 nm), which were believed to enhance the stability of LiMn2O4cathodes.