Oxidation behaviour of ferrous alloys used as interconnecting material in solid oxide fuel cells

Under operating conditions in the solid oxide fuel cell (SOFC), metallic interconnect plates form electrically insulating or poor-conducting oxide scales (e.g. Cr₂O₃, Al₂O₃) at their surface which increase the contact resistance from one fuel cell membrane to the next. In order to minimize electric losses in a fuel cell stack, the formation of oxide scales on the interconnect surface must either be prevented or the oxide scale formed must have sufficient electrical conductivity. In the present work, investigations were carried out on the corrosion behaviour of different FeCrAl and FeCrMn alloys, some of which were coated with nickel (Ni). Information about ageing of these alloys on the anode side of the fuel cell was obtained by means of contact resistance measurements and scanning electron microscopy. The results reveal that FeCrMn(LaTi) alloys and Ni-coated interconnects exhibit low ageing rates and are thus suitable for use on the anode side of SOFCs.     

Thermal conductivity and thermal contact conductance studies on Al2O3/Al–AlN metal matrix composite

Al₂O₃/Al–AlN is a metal matrix composite (MMC) used for making heat sink of electronic devices. This paper presents the detailed investigations carried out on thermal contact resistance across this MMC contact in vacuum at different contact pressures. The experimental results are compared with the theoretical models available in the literature for metallic contacts and they are found to be in good agreement with each other.     

Composite endplates with pre-curvature for PEMFC (polymer electrolyte membrane fuel cell)

A conventional PEMFC (polymer electrolyte membrane fuel cell) stack is composed of multiple stack composed of GDL (gas diffusion layer), MEA (membrane electrode assemblies), and bipolar plates sandwiched in between two thick metallic endplates tightened by bands or tie-bolts as to maintain proper contact pressure on its active area and gasket interface. The proper contact pressure distribution in a stack offers low contact resistance for high energy efficiency and fluid leakage prevention as well. For which, the endplates should have proper structural stiffness.     

Method for reducing contact resistivity of carbon nanotube-containing epoxy adhesives for aerospace applications

Bonded joints prepared with conductive epoxy adhesive based on carbon nanotubes (CNTs) display an unusually high resistance mainly due to the high contact resistivity at the adhesive metal interface. A new method is proposed to reduce efficiently the contact resistivity by forcing a controlled amount of electric current through the bonded joint. Current treatment at 0.5 A/cm² of current densities for 30 s typically reduces to up to 10 times the contact resistivity. Apart from noble metals, all other metals are usually covered by a more or less conductive oxide layer. This oxide layer is the main cause for high contact resistivity. During the current treatment large gradients of electric field developed around the highly curved CNTs are capable of breaking down locally the oxide layer generating conductive channels. Static shear strength and fatigue resistance of the bonded joint are not affected by the current treatment.     

Contact resistance simulations and measurements of MgB2–MgB2 lap joints

Resistive joints are found in many systems using superconductors. Joints are used to connect the superconductor to a normal conducting current terminal or to connect two superconductors, for example in pancake type windings. Knowing the resistance between the contacts is important in studying the heat balance of a superconducting system. We performed several experiments with MgB₂–MgB₂ lap joints to determine the relation between the contact resistance and solder joint length. Also, the effect of the outer sheath material on the contact resistance was studied. To support the experiments, a computational model using Finite Element Method was created. The measured and computed results showed adequate correlation. In the experiments, the soldered joint length was varied from 3 to 20 mm resulting in contact areas between 2.5 and 16.6 mm². The results indicated that the outer sheath material has significant effect on the contact resistance. For a Monel sheathed conductor the measured contact resistances varied between 4 and 16 μΩ and if a copper sheath was used, the resistances were an order of magnitude smaller.     

On the Applicability of Single-Walled Carbon Nanotubes as VLSI Interconnects

This paper presents a comprehensive study of the applicability of single-walled carbon nanotubes (SWCNTs) as interconnects in nanoscale integrated circuits. A detailed analysis of SWCNT interconnect resistance (considering its dependence on all physical parameters, as well as factors affecting the contact resistance), the first full 3-D capacitance simulations of SWCNT bundles for realistic very large scale integration (VLSI) interconnect dimensions, and a quantitative evaluation of the importance of inductive effects in SWCNT interconnects are presented. The applicability of carbon nanotube (CNT) based vias (vertical interconnects)-the most realizable CNT interconnects in the current state of the art-is addressed for the first time. It is shown that CNT interconnects can provide 30%-40% improvement in the delay of millimeter-long global interconnects. The applicability of CNT monolayers as local interconnects is found to be much more limited than that reported in the prior literature. Dense CNT bundle global interconnects are shown to offer a 4times reduction in power dissipation while achieving the same delay as optimally buffered Cu interconnects at the 22 nm node. This power saving increases to 8times at the 14 nm node. Furthermore, 3-D finite-element electrothermal simulations show that CNT bundles used as vias in between Cu metal layers can provide large improvement in metal interconnect lifetime by lowering the temperature of the hottest interconnects.     

Measuring the electrical resistivity and contact resistance of vertical carbon nanotube bundles for application as interconnects

Carbon nanotubes (CNT) are known to be materials with potential for manufacturing sub-20 nm high aspect ratio vertical interconnects in future microchips. In order to be successful with respect to contending against established tungsten or copper based interconnects, though, CNT must fulfil their promise of also providing low electrical resistance in integrated structures using scalable integration processes fully compatible with silicon technology. Hence, carefully engineered growth and integration solutions are required before we can fully exploit their potentialities. This work tackles the problem of optimizing a CNT integration process from the electrical perspective. The technique of measuring the CNT resistance as a function of the CNT length is here extended to CNT integrated in vertical contacts. This allows extracting the linear resistivity and the contact resistance of the CNT, two parameters to our knowledge never reported separately for vertical CNT contacts and which are of utmost importance, as they respectively measure the quality of the CNT and that of their metal contacts. The technique proposed allows electrically distinguishing the impact of each processing step individually on the CNT resistivity and the CNT contact resistance. Hence it constitutes a powerful technique for optimizing the process and developing CNT contacts of superior quality. This can be of relevant technological importance not only for interconnects but also for all those applications that rely on the electrical properties of CNT grown with a catalytic chemical vapor deposition method at low temperature.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

An investigation of contact resistance between carbon fiber/epoxy composite laminate and printed silver electrode for damage monitoring

An addressable conducting network (ACN) enables the structural condition to be monitored by the electrical resistance between electrodes on surface of CFRP (carbon fiber reinforced polymer) structure. To improve the reliability of ACN for damage detection, the contact resistance between the electrodes and CFRP laminates needs to be minimized. In this paper, the silver nanoparticles electrodes were fabricated via printed electronics techniques on CFRP composite. The contact resistance between the silver electrodes and CFRP was measured with respect to various fabrication conditions such as the sintering temperature of silver nanoink and the surface roughness of CFRP laminates. The interfaces between silver electrode and carbon fibers were observed using scanning electron microscope (SEM). From the study, it was found that the lowest contact resistance of 0.3664 Ω could be achieved when the sintering temperature of the silver nanoink and surface roughness were 120 °C and 230 nm, respectively.     

Liquid sensing properties of fibres prepared by melt spinning from poly(lactic acid) containing multi-walled carbon nanotubes

Poly(lactic acid) (PLA)/multi-walled carbon nanotube (MWNT) composites were melt spun with different take-up velocities (max. 100 m/min) to obtain electrically conductive fibres. The incorporation of MWNT contents between 0.5 and 5.0 wt.% was realised in a previous melt mixing process using twin-screw extrusion. The relative resistance change of the fibres caused by contact with different solvents (water, n-hexane, ethanol, methanol) and solvent concentrations was used as liquid sensing response, whereas the time dependent resistance was recorded during immersion and drying cycles. Transmission electron microscopy and Raman spectroscopy indicated enhanced orientation of MWNT along the fibre axis with take-up velocity, resulting in decreased sensitivity during solvent contact. Additionally, sensitivity decreased as the weight content of MWNT increased and was furthermore dependent on the characteristics of used solvents. In context with the targeted application of leakage detection, fibres with low MWNT amount and low draw down ratio (as extruded fibres with 2 wt.% MWNT) are suitable, as they showed relative resistance changes of up to 87% after 10 min immersion in methanol even if the recovery upon drying was suppressed significantly.     

Indium gallium zinc oxide (IGZO)-based Ohmic contact formation on n-type gallium antimony (GaSb)

In this paper, Ohmic-like contact on n-type GaSb with on/off-current ratio of 1.64 is presented, which is formed at 500 °C by inserting IGZO between metal (Ni) and GaSb. The resulting Ohmic contact is systematically investigated by TOF-SIMS, HSC chemistry simulation, XPS, TEM, AFM, and J–V measurements. Two main factors contributing to the Ohmic contact formation are (1) InSb (or InGaSb) with narrow energy bandgap (providing low electron and hole barrier heights) formed by In diffusion from IGZO and Sb released by Ga oxidation, and (2) free Sb working as traps that induces tunneling current.     

平板型SOFC合金连接板的研究进展

从材料设计和表面处理方面综述了合金材料作为连接板的研究近况。合金连接板具有高的电子传导和热传导性,成本低,加工性能优良,合金材料表面氧化膜生长速率适中。经过表面处理的合金,在工作过程中涂层与基体发生反应生成更抗氧化的保护层,面电阻率进一步降低,具有很好的应用前景。     

Electrochemical behavior of a lead-free SnAg solder alloy affected by the microstructure array

The aim of this study is to evaluate the electrochemical corrosion behavior of a Sn–Ag solder alloy in a 0.5 M NaCl solution at 25 °C as a function of microstructural characteristics. Different microstructure morphologies, which can be found in Sn–Ag solder joints and that are imposed by the local solidification cooling rate, are evaluated and correlated to the resulting scale of the dendritic matrix and the morphology of the Ag₃Sn intermetallic compound. Cylindrical metallic molds at two different initial temperatures were employed permitting the effect of 0.15 °C/s and 0.02 °C/s cooling rates on the microstructure pattern to be experimentally examined. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters. It was found that higher cooling rates during solidification are associated with fine dendritic arrays and a mixture of spheroids and fiber-like Ag₃Sn particles which result in better corrosion resistance than coarse dendrite arrays associated with a mixture of fibers and plate-like Ag₃Sn morphologies which result from very slow cooling rates.     

Investigation of mechanical and tribological behaviors of multilayer graphene reinforced Ni3Al matrix composites

Multilayer graphene (MLG) shows an attractive prospect for the demanding engineering applications. This paper reports the mechanical and tribological properties of MLG reinforced Ni₃Al matrix composites (NMCs) under dry sliding at varying sliding speed. The hardness and elastic modulus of the NMCs are significantly influenced with MLG content. It is found that the hardness and elastic modulus of the NMCs are found to be increased by increasing MLG content up to 1.0 wt.%, while decreased when MLG content is above 1.0 wt.%. Tribological experiments suggest that MLG can dramatically improve the wear resistance and decrease the friction coefficient of the NMCs. Such marked improvement of wear resistance is attributed to the reinforcing mechanisms of MLG, such as crack deflection and pull-out, and reduction of friction coefficient is related to the formation of a tribofilm on the sliding contact surface.     

Oxidation characterization of FeAl coated 316 stainless steel interconnects by high-energy micro-arc alloying technique for SOFC

Fe-based alloys have been extensively evaluated and considered as outstanding metallic interconnect materials for fuel cell. High-energy micro-arc alloying technique has been determined to be a feasible method of producing a consistent and dense FeAl intermetallic coating on 316 stainless steel substrate. The coating had an average thickness of about 50 μm and grain size was significantly refined. When exposed at 800 °C, 900 °C and 1000 °C in air after 100 h, FeAl coating on 316 SS substrate exhibited better high temperature oxidation resistance than electrode materials due to the conversion of non-protective Fe-rich scale into protective Al-rich one. FeAl intermetallic coating deposited by HEMAA will be available as interconnects for SOFC at very low costs.     

Morphological evolution of edge-hillocks on single-crystal films having anisotropic drift-diffusion under the capillary and electromigration forces

The morphological evolution of hillocks at the unpassivated sidewalls of single-crystal metallic thin film interconnects is investigated via computer simulations using the free-moving boundary value problem. The effect of drift-diffusion anisotropy on the development of surface topographical scenarios is fully explored under the action of electromigration and capillary forces, utilizing numerous combinations of the surface texture, the drift-diffusion anisotropy and the direction of the applied electric field. The simulation studies yield analytical relationships for the velocity of the surface solitary waves and the drift velocity of electromigration-induced internal voids as a function of the applied current densities, which contain intrinsic and structural properties of the single-crystal thin films. The threshold value of the applied current density, above which electromigration-induced internal voids can be formed and may cause the catastrophic failure of interconnects by breaching, also appears explicitly in this relationship.     

Characterization of electrochemically deposited Cu-Ni black coatings

Electrochemically deposited Cu-Ni black coatings on molybdenum substrate from ethylenediaminetetraacetic acid (EDTA) bath solution are shown to exhibit good optical properties (α=0.94, ε = 0.09). The deposit is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Cu is present in metallic and +2 oxidation states in the as-prepared Cu-Ni black coating, whereas Ni²⁺ as well as Ni³⁺ species are observed in the same coating. Cu and Ni are observed in their metallic state after 10 and 20 min sputtering. X-ray initiated Auger electron spectroscopy (XAES) of Cu and Ni also agrees well with XPS investigations.     

Bactericidal effects of different silver-containing materials

The evaluation of the bactericidal effect of different silver-containing materials where silver is available as Ag⁺ (silver nitrate and different silver-exchanged zeolites), as metallic Ag⁰ (commercial silver nanoparticles) or as oxide (silver (I) oxide) was carried out in order to elucidate the importance of the bioavailability of silver (i.e., as free ions, metallic particles, combination of them, clusters, complexes, partially soluble or insoluble salts, etc.) on its bactericidal action.For the different materials tested, their bactericidal effect is ordered in the following sequence: AgNO₃ > Ag-ZSM-5 > Ag₂O > commercial silver-exchanged zeolite (granular) > commercial silver-exchanged zeolite (pellets) > Ag nanoparticles. In general, as the content of bioavailable ionic silver increases, the biocidal effectiveness of the corresponding silver-releasing material increases too.     

Experimental study of high electric current effects in carbon/epoxy composites

Electrical and thermal behavior of the carbon fiber-reinforced epoxy composites subjected to relatively high (up to 75 A) steady electric currents is studied. A fully automated experimental setup for real time measurements of the electric current, resistance, voltage, and temperature in carbon fiber-reinforced epoxy matrix composites has been developed. A series of electrical characterization tests on IM7/977-3 unidirectional and symmetric cross-ply composite laminates have been performed and the effects of electric current magnitude and duration, electrical resistance, and associated thermal effects have been investigated. It is determined that electrical resistance exhibits time-dependent behavior. It is also found that application of an electric current leads to a significant temperature rise in the composites that is a result of the intense Joule heat produced in the electrically conductive carbon fibers as well as in the composite-electrode contact.