Low Electrical Contact Resistance Research Articles

Low-loss asymmetric strip-loaded slot waveguides in silicon-on-insulator

We report on low-loss asymmetric strip-loaded slot waveguides in silicon-on-insulator fabricated with 248 nm photolithography. Waveguide losses were 2 dB/cm or less at wavelengths near 1550 nm. A 40 nm strip-loading allows low-resistance electrical contact to be made to the two slot arms. The asymmetric design suppresses the TE1 mode while increasing the wavelength range for which the TE0 mode guides. This type of waveguide is suitable for building low insertion-loss, high-bandwidth, low drive-voltage modulators, when coated with an electro-optic polymer cladding.

Friction and contact resistance of nanocomposite Ti–Ni–C coatings

Ceramic nanocomposite coatings in the Ti–Ni–C were deposited using PVD and studied with respect to tribological properties and contact resistance. It was shown that coatings could be deposited combining of a low contact resistance and a low friction coefficient against silver, making them suitable for use in high performance electrical contacts. Nine coatings with different amounts of C and Ni were deposited. Coatings on flat Ni plated copper substrates were tested in a tribological ball-on-disc setup against ball bearing steel balls. Depending on primarily the amount of carbon the coatings showed very different friction coefficient and wear rate. The coatings were also deposited on cylindrical Ni plated copper substrates. Using geometrically identical silver plated cylinders as counter surface these were evaluated in a test setup better resembling a real life electrical contact. For most coatings a low electrical contact resistance was measured. The evolution of friction coefficient and contact resistance was correlated to wear marks and contact tracks, with their generated tribofilms, as examined after testing using electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy.

Oxidation behavior and electrical property of ferritic stainless steel interconnects with a Cr–La alloying layer by high-energy micro-arc alloying process

Chromium volatility, poisoning of the cathode material and rapidly decreasing electrical conductivity are the major problems associated with the application of ferritic stainless steel interconnects of solid oxide fuel cells operated at intermediate temperatures. Recently, a novel and simple high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO 3-based coatings for the type 430 stainless steel interconnects using a LaCrO 3–Ni rod as deposition electrode. In this work, a Cr–La alloying layer is firstly obtained on the alloy surface by HEMAA using Cr and La as deposition electrode, respectively, followed by oxidation treatment at 850 °C in air to form a thermally grown LaCrO 3 coating. With the formation of a protective scale composed of a thick LaCrO 3 outer layer incorporated with small amounts of Cr-rich oxides and a thin Cr 2O 3-rich sub-layer, the oxidation rate of the coated steel is reduced remarkably. A low and stable electrical contact resistance is achieved with the application of LaCrO 3-based coatings, with a value less than 40 mΩ cm 2 during exposure at 850 °C in air for up to 500 h.

LaCrO 3-based coatings deposited by high-energy micro-arc alloying process on a ferritic stainless steel interconnect material

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO 3-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO 3-based coating using an electrode rod of LaCrO 3–20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO 3-based coatings showed a three-layered microstructure with a NiFe 2O 4 outer layer, a thick LaCrO 3 sub-layer and a thin Cr 2O 3-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.

Fabrication of Metal Matrix Composites Based on Stainless Steel Having Low Electrical Contact Resistance by Powder Metallurgy

Metal matrix composite materials having low electrical contact resistance based on 316L stainless steel (STS) matrix alloy with ZrB2 or TiB2 particles were fabricated for PEMFC separator by powder metallurgy (PM). The addition of the boride particles remarkably enhanced electrical conductivity on the surface compared to commercial STS alloys. SEM observation on the surface of the samples revealed that the boride particles were protruded out of the matrix surface and the particle density existing on the surface increased with increasing the boride content, causing increase of the total contact area between the conductive particles and gas diffusion layer. The ICR of the samples also decreased with increasing the boride content resulted from the increased contact area. The enhanced electrical property is possibly attributed to the conductive particles protruded out of the matrix surface acting as electrically conductive channels through the passive film formed on the stainless steel matrix.

Fabrication of Metal Matrix Composites Based on Stainless Steel Having Low Electrical Contact Resistance by Powder Metallurgy

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Optimal deposition conditions of TiN barrier layers for the growth of vertically aligned carbon nanotubes onto metallic substrates

Plasma enhanced chemical deposition (PECVD) has proven over the years to be the preferred method for the growth of vertically aligned carbon nanotubes and nanofibres (VACNTs and VACNFs, respectively). In particular, carbon nanotubes (CNTs) grown on metallic surfaces present a great potential for high power applications, including low resistance electrical contacts, high power switches, electron guns or supercapacitors. Nevertheless, the deposition of CNTs onto metallic substrates is challenging, due to the intrinsic incompatibility between such substrates and the metallic precursor layers required to promote the growth of CNTs. In particular, the formation of CNT films is assisted by the presence of a nanometric (10–100 nm) monolayer of catalyst clusters, which act as nucleation sites for CNTs. The nanometric character of the precursor layer, together with the high growth temperature involved during the PECVD process (∼700 °C), strongly favours the in-diffusion of the catalyst nanoclusters into the bulk of the metallic substrate, which results in a dramatic reduction in the nucleation of CNTs. In order to overcome this problem, it is necessary to coat the metallic substrate with a diffusion barrier layer, prior to the growth of the catalyst precursor. Unlike other conventional ceramic barrier layers, TiN provides high electrical conductivity, thus being a promising candidate for use as barrier material in applications involving low resistance contacts. In this work we investigate the anti-diffusion properties of TiN sputtered coatings and its potential applicability to the growth of CNTs onto copper substrates, using Fe as catalyst material. The barrier and catalyst layers were deposited by magnetron sputtering. Auger electron spectroscopy was used to determine the diffusivity of Fe into TiN. Morphological characterization of the CNTs coatings was performed on scanning and transmission electron microscopes. Raman spectroscopy and x-ray diffraction were employed to establish a correlation between TiN characteristics and deposition parameters.

Electrically Conductive Thin Films Prepared from Layer‐by‐Layer Assembly of Graphite Platelets

AbstractLayer‐by‐layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/ββ‐methacryl‐oxyethyl‐trimethyl‐ammonium copolymer as the cationic binder. Nanoparticle zeta (ζζ) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field‐emission scanning electron microscopy and atomic‐force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self‐assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21 nm. Thin‐film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6 mΩ cm−2 at 173 nm to 3.5 mΩ cm−2 at 276 nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly.

Silica Nanoparticle Layer-by-Layer Assembly on Gold

Layer-by-layer (LBL) assembly of silica nanoparticles is investigated as a means of controlling the surface wetting properties of gold electroplated onto 316 L stainless-steel substrates while maintaining a low electrical surface contact resistance. The strong polyelectrolyte acrylamide/beta-methacryl-oxyethyl-trimethyl-ammonium copolymer is used as the cationic binder. The impact of silica nanoparticle zeta (zeta) potential for a range of -37.1 to 5.9 mV in the thickness, wettability, and contact resistance of the final LBL-assembled coatings is presented. The zeta potential is varied by altering both the pH and alcohol (ethanol) content of the silica suspensions and polymer suspension, consistent with the predictions of the Debye-Huckel equation. Nanoparticle adsorption is found to occur rapidly, with surface coverage equilibration obtained after only 1 min and uptake that is nearly linear with respect to the number of bilayers deposited. An increase in the absolute value of the (negative) zeta potential in the silica suspension is found to increase the bilayer thickness to an average value as high as 82% of the individual nanoparticle diameter for the smaller nanoparticles investigated, suggesting that nearly complete surface coverage may be achieved after the application of only a single nanoparticle-polymer bilayer (a coating thickness as low as 15.6 nm) and that nanoparticle adsorption is enhanced by electrostatic attraction between substrate and adsorbate. Counterintuitively, a more porous bilayer structure is observed if the zeta potential of the previously deposited nanoparticles is increased while the substrate is immersed in the cationic copolymer suspension, suggesting that copolymer adsorption in inhibited by substrate-solvent interactions. Wetting measurements demonstrate that silica LBL assembly results in a substantial reduction in contact angle from 84 degrees on the bare substrate surface to as low as 15 degrees after the application of a single bilayer and 7 degrees after the application of eight bilayers. A monotonic increase in coating contact resistance is observed with an increase in the thickness with a characteristic volumetric electrical through-plane resistivity of as low as 1.63 kOmega.cm obtained from contact resistance measurement.

Development of bipolar plates with different flow channel configurations for fuel cells

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.

Development of low resistance electrical contacts for thermoelectric devices based on n-type PbTe and p-type TAGS-85 ((AgSbTe2)0.15(GeTe)0.85)

Diffusion bonded very low resistance electrical contacts (specific contact resistance <10 µΩ cm2) have been developed on n-type PbTe and p-type TAGS-85 ((AgSbTe2)0.15(GeTe)0.85) thermoelements. Thermoelectric devices (TEDs) having two p–n couples in series (each of four elements having 7.5 mm diameter) generated an output power of 1.2 W (at an operating current of ∼17 A) at hot side temperature Th = 500 °C and a temperature difference ΔT = 410 °C. Internal resistance of the devices was found to increase linearly with an increase in the number of thermoelements. The efficiency of developed TEDs was found to be 6%. The devices have been operated continuously for more than 8 months in air without any degradation of the output power.

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate.

Integrating nanowires with substrates using directed assembly and nanoscale soldering

This paper describes a new methodology for integrating nanowires with micropatterned substrates using directed assembly and nanoscale soldering. Nanowires containing ferromagnetic nickel segments were fabricated by electrodeposition in nanoporous membranes. The nanowires were released by dissolution of the membrane and subsequently aligned relative to micropatterned substrates using magnetic field-directed assembly. After assembly, the wires were permanently bonded to the substrates using solder reflow to form low-resistance electrical contacts. This is the first demonstration of the use of nanoscale solder reflow to form low-resistance electrical interconnects between nanowires and substrates, and we demonstrated the utility of the strategy by fabricating a nanowire-based functional analog integrator.

Epitaxial growth of InP nanowires on germanium.

The growth of III-V semiconductors on silicon would allow the integration of their superior (opto-)electronic properties with silicon technology. But fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains have prevented the epitaxial integration of III-V with group IV semiconductors. Here we demonstrate the principle of epitaxial growth of III-V nanowires on a group IV substrate. We have grown InP nanowires on germanium substrates by a vapour-liquid-solid method. Although the crystal lattice mismatch is large (3.7%), the as-grown wires are monocrystalline and virtually free of dislocations. X-ray diffraction unambiguously demonstrates the heteroepitaxial growth of the nanowires. In addition, we show that a low-resistance electrical contact can be obtained between the wires and the substrate.

Copper–titanium thin film interaction

Interaction between 5 μm thick copper and 50 nm thin titanium films was investigated as a function of annealing temperature and time using MeV 4He+ Rutherford backscattering, X-ray diffraction and dynamic Secondary Ion Mass Spectrometry. Samples were made by depositing 10 nm of titanium on a PECVD silicon oxynitride, followed by 50 nm of titanium nitride and 50 nm of titanium in the said order. In the same system 100 nm of copper were subsequently sputtered; finally 5 μm of copper were grown by electroplating. This complex structure was chosen in order to investigate the possibility of using copper interconnects also in power devices. To investigate the composition and growth of Ti–Cu compound on the buried interface, it was necessary to develop a special procedure. The results of the investigation show the formation of a laterally non-uniform layer of TiCu4, which is presumably preceded by the formation of CuTi. The growth of the compound is kinetically controlled by means of a diffusion coefficient having 1.7 eV activation energy and a 5×10−2 cm2/s pre-exponential factor. The formation of a titanium–copper compound ensures a reliable and low resistance electrical contact especially at the vias. The Ti/TiN/Ti acts efficiently as a sacrificial and inert diffusion barrier. No copper was detected on the silicon oxynitride surface even after a 20-min 500 °C heat treatment.

Vacuum Heated Electroless Nickel Plated Contacts

Electroless nickel plating has been tried on steel or brass substrates. By selected conditions of heat treatment in a high vacuum environment the plating can produce chromium equivalent hardness without the effluents of the hard chromium plating process. The resulting surfaces were examined and characterized under an optical and a scanning electron microscope. X-ray diffraction analysis was also performed to investigate recrystallization effects. The fabricated contact materials were also tested under corrosion conditions and linear polarization measurements were performed. To exploit possible utilization of produced coatings as electrical contact or connector materials, semispherical nickel plated steel joints were tested under the simultaneous application of mechanical fretting and a low-voltage electrical load. Their contact resistance was monitored during 20,000 cycle tests. The results show that after a 2-min heat treatment under 800/spl deg/C in a high vacuum environment, the plating acquires a crystalline structure with microhardness exceeding 1100 HV and a very good adhesion to the substrate material, without deteriorating its corrosion wear properties. In addition they exhibit a low and stable electrical contact resistance when operating in adverse environments i.e. fretting conditions or corrosive atmosphere.

Bismuth nanowires for potential applications in nanoscale electronics technology.

Nanowires of bismuth with diameters ranging from 10 to 200 nm and lengths of 50 microm have been synthesized by a pressure injection method. Nanostructural and chemical compositional studies using environmental and high resolution transmission electron microscopy with electron stimulated energy dispersive X-ray spectroscopy have revealed essentially single crystal nanowires. The high resolution studies have shown that the nanowires contain amorphous Bi-oxide layers of a few nanometers on the surface. In situ environmental high resolution transmission electron microscopy (environmental-HRTEM) studies at the atomic level, in controlled hydrogen and other reducing gas environments at high temperatures demonstrate that gas reduction can be successfully applied to remove th oxide nanolayers and to maintain the dimensional and structural uniformity of the nanowires, which is key to attaining low electrical contact resistance.

Integration of carbon aerogels in PEM fuel cells

We prepared resorcinol–formaldehyde (RF) aerogel films with a high molar ratio of resorcinol/catalyst (R/C) of about 1500 and a low mass ratio of 30%. At the onset of the gelation process, organic fibers or carbon fiber fleece were added to the sol in order to increase the mechanical stability. Upon pyrolysis the RF-aerogel as well as the organic fibers were transformed into porous nanostructured carbon. The thickness of the films was about 500 μm and below. As previously shown, the addition of fibers modifies the gelation and thus the structure of the carbon network as well as the particle and pore sizes in characteristic ways. To investigate this influence, samples with different fiber types and contents were prepared. Scanning electron microscopy was used to determine the particle and pore sizes. The largest pore sizes obtained were in the range of several microns. The highest electrical conductivity reached was about 28 S/cm for a sample with almost 80% porosity. Tests of single cells prepared with carbon aerogel sheets show that films with a rather coarse structure in the interior and a micron thin fine structured top and bottom layer are most suitable as gas diffusion electrodes in polymer electrolyte membrane (PEM) fuel cells. The top and bottom layer promote the formation of low resistance electrical contacts between the gas diffusion electrodes and the polymeric membrane as well as the current collector.

High-field electrical transport in single-wall carbon nanotubes

Using low-resistance electrical contacts, we have measured the intrinsic high-field transport properties of metallic single-wall carbon nanotubes. Individual nanotubes appear to be able to carry currents with a density exceeding 10(9) A/cm(2). As the bias voltage is increased, the conductance drops dramatically due to scattering of electrons. We show that the current-voltage characteristics can be explained by considering optical or zone-boundary phonon emission as the dominant scattering mechanism at high field.

Low Temperature (300°C) Formation of Thermodynamically Stable NiSi2 Contacts to SiC

ABSTRACTThermodynamically stable, low specific contact resistance electrical contacts to SiC are essential to the application of such devices in high power or high temperature applications. However, thermal budgets impose processing constraints due to interfacial reactions and dopant profiles during device fabrication. Nickel disilicide (NiSi2) is useful for metallization of SiC, as stable contacts can be formed on SiC by reacting a fugitive layer of amorphous silicon (a-Si) with sputtered nickel at 300°C. NiSi2 forms directly upon reacting nickel with a-Si; this is not the case when nickel is reacted with crystalline silicon, where more nickel-rich silicides form first. Specific contact resistances as low as 5.6×10−5 Ωcm2 have been measured using NiSi2 circular TLM contacts so formed on APCVD 3C-SiC thin films deposited on a silicon wafer.