Key Engineering Materials Vols. 656-657

Abstract: This study examines two things about a dye sensitized solar cell (DSSC) to improve power conversion efficiency. One is how to make ZnO-coated TiO2 electrode. The other is how to make carbon nanotube (CNT) electrode. First, we considered the process of making the ZnO-coated TiO₂ electrode of the DSSC. This ZnO coating of the DSSC is important for the increase of power conversion efficiency. The fabrication method of the ZnO-coated TiO₂ electrode was simple dip coating. This method uses the immerse of the zinc acetate dehydrate [Zn (CH₃COO₂)・2H₂O] solution. This method can make the cheap ZnO-coated TiO₂ electrode. However, this method has a slightly negative effect, which is filling in holes of the porous TiO₂ layer. We tried to improve this negative effect. We changed the concentration of a zinc acetate dehydrate solution from low to high. Also, we changed the immersing time of the zinc acetate dehydrate solution. We did the control of the band gap of ZnO-coated TiO₂ electrode of DSSC for increasing power conversion efficiency. Second, we substituted CNT for counter electrodes to improve the performance of DSSC. As a manufacture method of CNT electrode, we used electrophoretic deposition (EPD). After that, we baked this CNT electrode and measured its specific surface area. We tried to improve specific surface area by changing baking temperature.

Abstract: A silicon substrate is the starting point of producing the semiconductor component, so that the quality of semiconductor substrate is very important during the VLSI fabrication. In this paper, we will evaluate the influence of MOS device characteristics under different oxygen impurities in silicon substrates. In the course of silicon substrate pulling process by Czochralski method, the defect and impurity will be existed; the oxygen atom will be induced substrate dislocations and affected the substrate quality. In this work, different oxygen doses will be used in wafer to study the impacts on MOS CV curve characteristic, interface trap charge characteristic, I_D-V_DS curve, I_D-V_GS curve, and threshold voltage behaviors of MOS devices.

Abstract: Porous materials, such as filters made of sintered metals, lagging materials, and fireproof materials, are utilized in various fields. The porosity changes the characteristics of the materials. For example, with heat-insulating foam, the higher the porosity, the greater is the insulation factor. However, increased porosity leads to a decline in mechanical properties. Thus, when using porous materials, analyzing the mechanical strength is necessary. We modeled a porous structure of sintered metal sample and estimated the Young's modulus using the numerical analysis software “ADVENTURE” and compared the estimated value with the experimental value. Also, we modeled the effect of porosity and pore diameter on the mechanical property of the material. From the results, the Young's modulus decreases with increases in porosity and pore diameter, as expected.

Abstract: In this work, a series of CaO-based materials for medium-high temperature (500-800 °C) CO₂ capture were synthesized from limestone by acidic treatment. In order to enhance its thermal stability, CaO-based materials were modified with Al(NO₃)₃. With various stirring time of 24, 48 and 72 hours, three types of CO₂ capture sorbent were synthesized, named LA24, LA48 and LA72, respectively. TGA results showed the CaO-based materials were activated in the CO₂ capture reaction of temperature range of 500-800 °C. LA24 exhibited the best CO₂ capture capacity of 53.8 wt % as well as stability of 93.2 % after 10 cycles test. Furthermore, all of the modified CaO-based materials showed good thermal resistance upon 900 °C. It was consistent with expectation by the contributions of alumina coexistence with calcium oxide. XRD results indicated that CaO dominated in the diffraction spectrum before capturing CO₂, and a side diffraction peak of Al₂O₃ was also observed. The characterization of CaO-based materials was also investigated by BET and SEM, respectively.

Abstract: The novel carbonate sorbents of Mg–Al–CO₃ and (Mg_3−x, Cu_x)–Al–CO₃, were synthesized by co-precipitation method with individual nitrate salt of metal ions under alkaline conditions. The synthetic sorbent was characterized by analysis techniques such as BET surface area analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental mercury capture experiments were carried out in a fixed-bed reactor including Hg permeation source, furnace, and Hg analyzer, which was conducted at temperature ranging from 30 to 300 ^o C. The major results showed that the surface area of material was significantly increased via incorporating Cu²⁺ into Mg–Al–CO₃, accordingly enhancing Hg retention capacity of sorbents. SEM imagines displayed the layer appearance of Mg/Al and Mg/Cu/Al sorbents. Crystalline analysis indicated lamella structure accompanied with metal oxides within materials. Mercury removal tests demonstrated that the breakthrough time increased with temperature by adding transition metals to Mg–Al–CO₃ as (Mg_3−x, Cu_x)–Al–CO₃. Hg uptake by the (Mg_3−x, Cu_x)–Al–CO₃ sorbent rapidly increased with elevated temperature up to 200 ^o C and reached the maximum capacity of 12.93 μg/g, and then gradually decreased after 300 ^o C. Surface area and unique properties of transition metals are the reason toward improving Hg capture sorbent. These results represent the feasibility of using such Hg sorbents for elemental mercury removal under elevated temperature conditions, and the detail mechanism is needed to be further studied.

Abstract: Silicon carbide (SiC) is widely employed as an abrasive material in aqueous media for sawing silicon ingot into individual wafers in photovoltaic industry. After a series of cutting, grinding and polishing operation, a mixture of substances (Cutting fluid, SiC, Si and small amount of magnetic metal) is produced as a form of slurry. The used SiC can be preferably recovered and reused for another application, rather than disposed of as waste. In this study, a pilot scale system (25 kg/h) is developed to extract SiC from photovoltaic industry abrasive slurry. The recovery system is composed of physical and chemical separation processes to remove silicon particles and magnetic materials which are dispersed in the slurry. X-ray diffraction analysis showed that purified powder is in the 6H-SiC structure and powder consists only of silicon carbide and has no residual silicon. It might be applied again in silicon ingot cutting or for other purposes which require this kind of ceramic material.

Abstract: WC–20 mol% SiC ceramics with added Cr₃C₂ were sintered at 1600°C with a resistance-heated hot-pressing machine. Dense WC–SiC ceramics containing 0.1–0.9 mol% Cr₃C₂ were obtained. Above 1.2 mol% Cr₃C₂, the relative density decreased with increasing Cr₃C₂ content. A small amount of a Nowotny-phase type (Mo₅Si₃C-type) product was formed by the addition of Cr₃C₂, and no Cr₃C₂-based solid solution was found. The WC–20 mol% SiC–Cr₃C₂ ceramics had very fine equiaxed granular WC grains because of inhibited grain growth of WC. The Young’s modulus of the WC–20 mol% SiC–Cr₃C₂ ceramics decreased with increasing Cr₃C₂ content because Cr₃C₂ has a much lower Young’s modulus than WC. Cr₃C₂ addition below 0.9 mol% increased the Vickers hardness from 20.9 to 23.0 GPa, but a larger added amount reduced the Vickers hardness. The hardness of the WC–20 mol% SiC–Cr₃C₂ ceramics and the WC grain size obeyed a Hall–Petch-like relationship, suggesting that the hardness was strongly controlled by the WC grain size. A higher fracture toughness, 6.4 MPa m^1/2, was obtained for the ceramics containing a small amount of Cr₃C₂ than for the binder-free WC. The addition of 0.1–0.3 mol% Cr₃C₂ improved the fracture toughness without reducing the hardness.

Abstract: The objective of this research work is to investigate the effect of Aluminum addition in cast nickel base superalloy grade Inconel-738 by vacuum arc melting process on microstructural modification and oxidation behavior at elevated temperatures of 900°C and 1000°C. The Al element, basically, could be added in cast nickel base superalloys in proper amount to form precipitated intermetallic phase with nickel atoms as gamma prime phase (γ’, Ni₃Al) to increase mechanical properties by blocking dislocation movements at elevated temperatures. Furthermore, Al can assist nickel base superalloy to form protective oxide film, Al₂O₃ for better oxidation resistance at very high temperatures (over 980°C). In this research, all casted samples of Inconel-738 with various Al additions for 1, 2 and 3 percent by weight were standard heat treated consisting of solution treating at 1125°C for 6 hours and following with precipitate aging at 845°C for 24 hours. The oxidation tests were carried out at temperatures of 900°C and 1000°C up to 110 hours. From all obtained results, it was found that the sample that has the most microstructural stability after long-term heating as simulated working conditions is Inconel-738 sample with 2%wt. Al addition. Furthermore, more Al addition had resulted in higher oxidation resistances for both testing temperatures.

Abstract: (Na_0.5K_0.5)NbO₃ with 0~5 mole% Bi_0.5(Na_0.95K_0.05)_0.5TiO₃ has been prepared following the conventional mixed oxide process. It can be concluded that the NKN-BNKT ceramics have orthorhombic structures in the case x≤ 0.02. With increasing BNKT content (x=0.03 to 0.05), however, the structure changes from orthorhombic to rhombohedral phase. Above results demonstrated that the MPB between orthorhombic and rhombohedral phases exits in the solid solution with the BNKT content of x=0.02. For 0.98(Na_0.5K_0.5)NbO₃-0.02Bi (Na_0.95K_0.05)TiO₃ ceramics, the electromechanical coupling coefficients of the planar mode k_p and the piezoelectric constant d₃₃ reach 0.28 and 155 p/CN, respectively, at the sintering of 1100 °C for 3 h. Dielectric and piezoelectric properties have maximum values at the sintering temperature of 1100 °C for 5 h. For 0.98(Na_0.5K_0.5)NbO₃-0.02Bi_0.5(Na_0.95K_0.05)_0.5TiO₃ ceramics, the electromechanical coupling coefficients of the planar mode k_p and the piezoelectric constant d₃₃ reach 0.30 and 170, respectively, at the sintering of 1100 °C for 5 h. The effect of prolonging the sintering time to the 0.98(Na_0.5K_0.5)NbO₃-0.02Bi_0.5(Na_0.95K_0.05)_0.5TiO₃ system is a helpful method on ceramic processing to improve densification and properties..