Key Engineering Materials Vols. 656-657

Abstract: The purpose of the present study is to evaluate the effect of the electroless Ni-P/diamond/graphene composite coating on the structure and surface hardness of 2024-T6 aluminum alloy as well as their effect on the corrosion and wear resistance of the alloy in 3.5 % NaCl solution. The electroless Ni-P plating solution was prepared by adding different size diamond (6-12 μm and 0.2 μm) and nanographene into the electroless Ni-P plating solution to obtain Ni-P/diamond, Ni-P/graphene and Ni-P/daimond/graphene composite coatings for comparison. Experimental results indicated that the Ni-P/diamond, Ni-P/graphene and Ni-P/daimond/graphene composite coatings can be successfully electroless deposited on anodized 2024-T6 aluminum alloy. The anodically oxidized films, that formed on the aluminum alloy using phosphoric acid as the electrolyte, was porous with high density of pores, and thus could enhance the adhesion of the composite coatings. The Ni-P/daimond/graphene hybrid coating had a higher hardness as well as better corrosion and wear resistance of 2024-T6 alloy in 3.5 wt.% NaCl solution as compared with other composite coatings. When the combination of nanographene and smaller diamond particles added this beneficial effect was significantly raised, especially the composite coating was further vacuum annealed at 400 °C for 24 h to obtain a more smooth and defect-free coating structure.

Abstract: Compounds such as GaN, ZnSe, and SiC are the compounds that currently hold the most potential in developing blue light-emitting diodes (LEDs) and blue laser diodes (LDs). Speaking of the physical property, the gallium nitride belongs to a direct bandgap material with an obviously super luminous efficiency; therefore, the gallium nitride has the dominate tendency than that of others materials. Although the gallium nitride has excellent physical properties, but in actually it is suffered many challenges during the manufacture process. Especially, it is extremely sensitive to the electrostatic discharge (ESD) threat. In other words GaN diodes generally exhibit very low anti-ESD capabilities when in HBM, MM reversed bias modes. These LEDs in the MM stress situation, its ESD immunity level usually is only about 50-V extremely low anti-ESD ability. Therefore, in this paper, GaN LED DUTs will be stressed and investigated under HBM and MM pulses bombardments, and the aim of this work is to describe a detailed investigation of the factors that limit the robustness of GaN-based LEDs under ESD transient events; finally they will provide some countermeasures in ESD reliability consideration.

Abstract: In situ TiO₂ nanofiber arrays have been successfully produced directly on a Ti-6Al-4V substrate by using thermal oxidation under a limited supply of oxygen. Their morphology, elemental composition, crystal structure, surface roughness and surface wettability were characterized by field-emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD), atomic force microscope (AFM) and contact angle goniometer, respectively. The results of material characterization studies revealed that TiO₂ nanofibers possessed greater surface roughness and wettability, as well as the degree of crystallinity. In vitro characterization have also been evaluated by using bovine articular chondrocytes on the resulting TiO₂ nanofibrous surface at different time points. Cell adhesion was observed qualitatively by using FESEM and cell proliferation was determined quantitatively by using AlamarBlue reduction assay. The results showed that the TiO₂ nanofibrous substrate triggers enhanced chondrocytes adhesion, proliferation, and production of extracellular matrix (ECM) fibrils compared to untreated substrate. These results suggest that the oxidation process produces a surface structure to which chondrocytes affinity, and thus this surface would has potential use in implants designed for cartilaginous applications.

Abstract: Lanthanum strontium manganite oxides with perovskite structure are widely employed materials for protective coatings on chromium-contained metallic interconnectors in the intermediated temperature solid oxide fuel cells (ITSOFCs). The application of protective coatings is used to decrease the growth of chromium oxide and the evaporation of chromium trioxide and chromium hydroxide from the surfaces of metallic interconnectors. In this study, La_0.8Sr_0.2MnO_3-δ (LSM) protective coatings are produced by the promising atmospheric plasma spraying (APS) technique on the substrates of Crofer 22 H, Crofer 22 APU and SS441 ferritic steels with or without pre-oxidation treatment. The substrates with pre-oxidation treatment were heated to 800°C and dwelled for 12 hrs in air before APS coating process. The cross-sectional micrographs show that the LSM coatings produced by APS technique are quiet dense without penetrating cracks. The XRD results identify that the LSM coatings produced by APS under 50 kW torch power reveal desired perovskite structure without any X-ray detectable second phase. After 600 hrs ageing in air at 800°C, the initial and final ASR values of the coated Crofer 22 APU sample with pre-oxidation treatment are 1.350 and 1.694 mΩcm², respectively. The measured ASR increasing rate is only about 0.573 μΩcm²/hr. Thus, LSM coating prepared by APS technique can dramatically decrease the growth of chromium oxide to protect the metallic interconnector and the generation of gaseous Cr-contained species to avoid cathode poisoning at the operating temperatures of ITSOFCs.

Abstract: In this study nanoTiO₂ particles were incorporated into the electroless plating solution to prepare Ni-P-TiO₂ composite coating on anodized AA7075 aluminum alloy to improve the wear and stress corrosion cracking resistance of the coated alloy in 3.5%NaCl solution. The anodized AA7075 aluminum alloy was also performed by a boiling water sealing treatment for comparison. The wear and stress corrosion cracking (SCC) characteristics were investigated using a self-designed block-on-ring machine and slow strain rate test. The effect of corrosion was evaluated by electrochemical polarization measurements. The surface morphology, element composition and surface hardness of the coating were analyzed by scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (EDS) and Vicker′s hardness tester. Experimental results indicated that after boiling water sealing treatment the resistance properties of the anodized AA7075 aluminum alloy were further improved. The anodizing treatment of AA7075 aluminum alloy gave a thick film with high porosity. The porous film efficiently improved the cohesion, adhesion and hardness of the electroless Ni-P composite coating. Therefore, the electroless Ni-P composite coating deposited on the anodized AA7075 aluminum alloy offered a superior wear, pitting corrosion and stress corrosion cracking resistance properties than both anodizing and sealing treatment. By comparison with Ni-P and Ni-P-TiO₂ coatings the incorporation of TiO₂ resulted in a more uniform and crack-free surface structure of the composite coating. This is responsible for the higher hardness, better wear, pitting corrosion and stress corrosion cracking resistance of the electroless Ni-P-TiO₂ coating.

Abstract: Cubic Boron Nitride (cBN) has wide application prospect in the field of high performance cutting tool, for its outstanding mechanical properties. In this paper, the cBN coating was synthesized on cement carbide by chemical vapor deposition nanocrystalline diamond interlayer in radio frequency magnetron sputtering system. The SEM, FTIR and AFM were used to investigate the microstructure, purity and morphology of the cBN film. The research was carried out on the effect of the deposition parameters on the cBN content in the film. High temperature and high pressure locally in the substrate is an important factor of transformation of cubic phase BN from hexagonal phase BN. There is a narrow optimization parameters window in cBN synthesis by ion bombardment assist radio frequency magnetron sputtering method. The good quality of cBN film can be obtained by a combination of moderate substrate bias voltage and gas pressure, high substrate temperature and a certain N₂ partial pressure.

Abstract: In this study, production processes for porous alumina, and the characteristics of the material, were investigated. Porous alumina was produced by a wet-shaping process in which air bubbles were introduced into the slurry. The feature of this production process is that many pores are produced by slip casting carried out using whipped slurry, where only the conditions of the slurry are adjusted. The advantage of this process is its simplicity. From the results, it is made clear that a green compact of porous alumina can be produced by changing the amount of solvent and binder, and also that a sintered compact of porous alumina can be produced by a low sintering temperature, such as 1473 K. The four point bending strength of porous alumina is about 515 MPa when the porosity is about 30 %. The excellent characteristics of the sintered compact of porous alumina are shown by the observation results of the fracture surface in this production process. The dense alumina body is sintered while maintaining the fine grains, and with the micro pores remaining in the grain boundary.

Abstract: The uniformity improvement of high deposition rate in hydrogenated amorphous silicon (a-Si:H) film deposited by electron cyclotron resonance chemical vapor deposition (ECR-CVD) is very essential for a large substrate in PV solar industry. In order to improve the uniformity in depositing thin film in large area, the auxiliary magnetic coils were designed and installed in ECR-CVD to modify the distribution of magnetic field. In addition, the dependence of the other ECR-CVD processing parameters such as resonance position, microwave power, working pressure, and substrate temperature were investigated. The results indicated that more uniform a-Si:H film could be obtained when working pressure was decreased. By using finite element analysis, it was found that location of turbo pump would impact gas flow field and this effect would become more significant at high pressure. Increasing microwave power, increasing horizontal gradient of the magnetic field to the substrate, and forming Cusp magnetic field could enhance ECR-CVD deposition uniformity greatly. However, the plasma location and substrate temperature were not major factors affecting a-Si:H film uniformity in ECR-CVD process. Finally, the optimal and the best 3.8% in uniformity could be achieved in 150mm diameter when the ratio of magnetic field strength at wafer edge to wafer center is 215%, working pressure is 1.5 mtorr, microwave power density is 4W/cm², and substrate temperature is 180°C.

Abstract: The study is concentrated on thermodynamic analyses of gas desulfurization process (deep removal of H₂S, COS, thiophene) by selected solid sorbents (ZnO, MnO, Ce_xO_y and La₂O₃) and on interferences caused by presence of hydrogen halides in a temperature range 500-1100 K. The results show that theoretically Ce₂O₃ and La₂O₃ are the best sorbents for sulfur compounds at temperatures over approx. 700 K. The Ce_xO_y, La₂O₃ and MnO based sorbents can suffer from significant interferences caused by higher concentrations of HCl and HF in gas phase. The thermodynamic equilibria suggest that removal of HCl (HF) by soda based sorbents at temperatures 650 – 850 K is practically without interferences from sulfur compounds. The common alkali carbonates are less suitable than the calcium based (Ca (OH)₂, CaCO₃) sorbents for deep removal of HF.

Abstract: An in situ technique for monitoring the growth rate and optical constants of the thin semiconductor layer by the normal-incidence reflectance is proposed. To demonstrate the feasibility of the proposed method, the variation of the air gap between two glasses is used to simulate growth system. We also used the spin coater to thin the thickness of the salad oil to test the performance of the measurement system. The experiments indicate that we can determine the thickness variation and optical constants of the test sample in real time.