Electrical Contact Applications Research Articles

Improving the contact resistance at low force using gold coated carbon nanotube surfaces

Investigations to determine the electrical contact performance under repeated cycles at low force conditions for carbon-nanotube (CNT) coated surfaces were performed. The surfaces under investigation consisted of multi-walled CNT synthesized on a silicon substrate and coated with a gold film. These planar surfaces were mounted on the tip of a PZT actuator and contacted with a plated Au hemispherical probe. The dynamic applied force used was 1 mN. The contact resistance (Rc) of these surfaces was investigated with the applied force and with repeated loading cycles performed for stability testing. The surfaces were compared with a reference Au–Au contact under the same experimental conditions. This initial study shows the potential for the application of gold coated CNT surfaces as an interface in low force electrical contact applications.

The Relationship Between Contact Resistance and Contact Force on Au-Coated Carbon Nanotube Surfaces Under Low Force Conditions

Carbon nanotube (CNT)-coated surfaces are investigated to determine the electrical contact performance under low force conditions. The surfaces under investigation are vertically aligned multiwalled CNTs formed on a silicon substrate and coated with an Au film. These planar surfaces are mated with a hemispherical Au plated probe mounted in a nanoindentation apparatus. The maximum contact force used is 1 mN. The contact resistance of these surfaces is investigated as a function of the applied force and is also studied under repeated loading cycles. The surfaces are compared with a reference Au-Au contact under the same experimental conditions and the results compared to established contact theory. The results show that the vertically aligned multiwalled CNT surface provides a stable contact resistance. This paper shows the potential for the application of CNT surfaces as an interface in low force electrical contact applications.

Tribological Performance of Microwave-Heat-Treated Copper–Graphite Composites

Copper–graphite composite is a tribological composite that can be used in sliding electrical contact applications requiring low friction and wear in addition to high electrical conductivity. The graphite powder (5 wt%) was mixed with the copper powder, and then composite was fabricated through powder metallurgy (P/M) route. P/M product generally requires secondary operations such as rolling, extrusion, etc. to improve their mechanical properties. Post-heat-treatment technique is also applicable to improve the properties of P/M components. Microwave-post-heat-treatment research studies are gaining momentum nowadays due to the improved quality of products with reduced time, energy, and associated cost. Microwave post-heat treatment of copper–graphite composites for different heat treating duration was carried out in a hybrid microwave heating setup. Microstructural studies were carried out using SEM with EDAX. Microwave-heat-treated samples exhibited reduced porosity, improved density, and hardness. In order to understand the friction and wear properties of microwave-heat-treated copper–graphite composites, pin-on-disk wear experiments were conducted. For comparison, untreated copper–graphite composites were also subjected to similar studies. Microwave-heat-treated samples exhibited reduced coefficient of friction and specific wear rate when compared to the untreated ones. The wear mechanism of untreated composites was observed to be plastic deformation characterized by large wear fragments, whereas the mechanism of heat-treated composite was delamination observed through peel off tribolayer.

Facet-Selective Epitaxial Growth of Heterogeneous Nanostructures of Semiconductor and Metal: ZnO Nanorods on Ag Nanocrystals

We demonstrate a new approach for synthesizing Ag-ZnO heterogeneous nanostructures in which single-crystalline ZnO nanorods were selectively grown on {111} rather than {100} facets of single-crystalline Ag truncated nanocubes. We have identified the fine structure of the Ag-ZnO heterostructures and proposed a mechanism indicating that structure match plays a critically important role in this type of facet-selective growth. These heterogeneous nanostructures are of special interest and have potential applications in electrical contacts, functional devices, biological sensors, and catalysis.

Gold–Carbon Nanotube Composite Plating Film Deposited Using Non-Cyanide Bath

We developed a gold–carbon nanotube (Au–CNT) composite plating film by electrodeposition. An effective additive was added into a non-cyanide CNT plating solution, affording an environmentally friendly method for fabricating an Au–CNT composite plating film using a basic solvent. The fabricated Au–CNT composite film was harder than the Au film by 1.4 times and showed a low friction coefficient. Thus, the Au–CNT composite film could be considered as a promising material for next-generation electrical contact applications.

Fabrication of Silver-Graphite Contact Materials Using Silver Nanopowders

Silver-graphite (AgC) composites are used in electrical switchgears as arcing as well as sliding contacts. AgC composite powders for electrical contact applications are conventionally prepared using micron-size silver powder. Present investigation is aimed at exploring the effect of nanosize silver powder, made by colloidal synthesis route, on the processing and properties of AgC contact materials. The AgC composite powders synthesized from micron-size and nanosize silver powders, respectively, are characterized for particle size distribution by dynamic light scattering technique, x-ray diffraction, and scanning electron microscopy. The bulk solid compacts produced by conventional powder metallurgy process of pressing, sintering, and repressing of AgC composite powders are evaluated for their density, microhardness, electrical conductivity, and microstructure. The study reveals that the use of nanosize silver powder not only leads to reduction in sintering temperature but also contributes in improving the properties of the AgC contact materials.

High-temperature mechanical and electrical properties of W-particle-reinforced Cu-matrix composites from improved electroless plated powders

Copper (Cu)-matrix composites combine the high electrical and thermal conductivities of Cu with the enhanced wear resistance and strength of the reinforcement material, and are therefore widely used for electrical contact applications in a diverse range of fields. In the current study, tungsten-reinforced copper (W—Cu) composite powders are fabricated using a novel electroless Cu plating process in which the homogeneity of the powder mixture is improved via the use of an ultrasonic agitation technique. The volume content of the tungsten particle reinforcement in the composite specimens is 3, 6, and 12 per cent, respectively, and standard powder metallurgical techniques are then applied to produce the final composite powders. The mechanical and electrical properties of the composite powders are evaluated at room temperature and at temperatures ranging from 100 to 300 °C. Fracture surfaces of the composite samples are ana-lysed using scanning electron microscopy. The results show that the mechanical properties of the W—Cu composite samples are significantly better than those of pure Cu specimens, particularly at higher temperatures. Furthermore, the addition of 3 vol% W particles to the W—Cu composite is found to yield a reduction of not more than 2 per cent in the electrical conductivity of the composite sample compared to that of a pure Cu specimen. Overall, the results indicate that the optimal mechanical and electrical properties are obtained by mixing pure Cu powder with 3 vol% of Cu-plated W particles with a mean dimension of 1 μm.

Impact of the nickel interlayer on the electrical resistance of tin–tin interface submitted to fretting loading

The behaviour of the tin/tin interface for electrical contact application is addressed in this paper. Two coating systems were studied: a bronze–tin coating system and a bronze–nickel–tin coating system. The transition amplitude δ t from partial slip (P.S) to gross slip (G.S), defining the domains of infinite and finite lifetime of the contact, was found using the variable–displacement–amplitude methodology (VDA). Constant–displacement–amplitude (CDA) tests were performed in order to determine the influence of the nickel interlayer on the performance of the electrical contact. The conducted analysis concluded that there is no influence of the nickel interlayer on the electrical endurance during G.S. However, the nickel interlayer, which serves as a diffusion barrier, eliminates the diffusion of copper through the tin coating; thus preventing the formation of copper oxides on the top surface. It concludes that the application of the nickel underlayer by maintaining high tin–tin friction coefficient extends the P.S domain and therefore increases the reliability of the electrical contact.

The Effects of Te and Misch Metal Additions on the Microstructure and Properties of Rapidly Solidified Ag-Sn-In Alloy for Electrical Contact Applications

High wear resistance with a stable contact resistance is a prerequisite for electrical contact applications. Although Ag−CdO has been widely used as an electrical contact material, there are intrinsic problems of forming large Cd oxide particles and environmental regulations against using Cd. Newly developed Ag−SnO2 alloys are considered good candidates to replace Ag−CdO alloys due to their stable and fine oxide formation capabilities. In addition, further improvements in performance are expected in Ag−SnO2 alloys by alloy modification and/or solidification processing to produce finer and stable oxide dispersions through internal oxidation. The effect of the addition of Te and misch metal, which function as oxide forming elements, on Ag−Sn−In ternary alloy was investigated. Up to 0.5 wt.% of Te and misch metals were added and rapidly solidified to maximize their effect on fine oxide formation in an Ag matrix. Resulting microstructural changes and properties were evaluated through electron microscopy, spectroscopy, and hardness measurements. The role of Te addition was to provide nucleation sites for complex oxides such as In2TeO6 phase and to ensure fine and well dispersed SnO2 oxide particles. A rapid increase in size was observed for both grain and oxide particles when Te content exceeded 0.3 wt.%. Misch metal addition, on the other hand, had a pronounced effect on grain size reduction of the Ag matrix, and was interpreted as a consequence of decreasing the latent heat of solidification. Maximum hardness was achieved at 0.3 wt.% misch metal addition. In both cases, hardness decreased rapidly at 0.5 wt.% addition and was attributed to the large grain size of the matrix, and also large oxide particles aggregated in the matrix grains.

Tribological and electrical study of fluorinated diazonium films as dry lubricants for electrical contacts

AbstractThe need to operate in extreme environmental conditions (ultra high vacuum, high temperatures, aerospatial, etc.) and the miniaturisation toward microelectromechanical system is demanding new materials in the field of low‐level electrical contacts lubrication. A dry and chemically immobilised layer avoiding the traditional wet lubricating fluids would have many advantages.We report here the first results on films obtained by the electrochemical reduction of different diazonium salts and their use as protective coatings able to lubricate the metallic surfaces of an electrical contact while preserving at the same time electrical conduction.The physicochemical properties of the films, elaborated from various diazonium salts, were studied by cyclic voltammetry and ATR FT‐IR spectroscopy. Atomic force microscope was combined with a home‐made wide‐range current measurement system to investigate simultaneously the mechanical and electrical interactions between the doped diamond conductive tips and the evaporated gold surfaces coated with the diazonium films. The analysis of the simultaneous mechanical and electrical properties of the films in a nanocontact situation is shown to bring much insight into the electrical contact application. Copyright © 2008 John Wiley & Sons, Ltd.

Bodycote Prize 2006: Best Technical/Scientific Paper Novel ceramic Ti–Si–C nanocomposite coatings for electrical contact applications

Nanocomposite coatings consisting of TiC nanocrystallites embedded in an amorphous SiC matrix (nc-TiC/a-SiC) have been produced using magnetron sputtering from a Ti3SiC2 target onto electrical contact components at a temperature below 300°C. The as deposited nanocomposites were employed as electrical contact materials. Results show that when contacted against Ag, they exhibit somewhat higher contact resistance than Ag against Ag, while preventing welding and potentially minimising wear. This is due to an adequate resistivity coupled with beneficial mechanical properties. The design of the nanoscale materials produced coatings with a relatively high nanoindentation hardness of 20 GPa, but with a ductile deformation behaviour. The ductility is explained by rotation and gliding of nc-TiC grains in the matrix. Therefore, the nanocomposite material can minimise problems with wear and welding while retaining low contact resistance. Consequently, this type of novel coating with appropriate mechanical properties offers a solution to problems in many electrical applications. Moreover, the use of physically vapour deposited coatings is promoted in some previously relatively unexploited areas of electrical contact applications.

Combustion synthesis of TiB2-based cermets: modeling and experimental results

TiB2-based cermets are prepared by combustion synthesis followed by a pressing stage in a granulate medium. Products obtained by combustion synthesis are characterized by a large remaining porosity (typically 50%). To produce dense cermets, a subsequent densification step is performed after the combustion process and when the reacted material is still hot. To design the process, numerical simulations are carried out and compared to experimental results. In addition, physical and electrical properties of the products related to electrical contact applications are evaluated.

Effect of Humidity on Growth of Oxide Film on Surface of Copper Contacts

Contact surfaces are exposed to the atmosphere in general applications. Therefore, gases in the atmosphere such as oxygen and H 2 O are adsorbed on and react with the contact surface. Products formed on the surface such as copper oxide films degrade contact resistance characteristics. This surface contamination is an important problem for electrical contact applications. The author has studied the effect of humidification on contact resistance characteristics. In this paper, the effect of humidity on the growth of an oxide film on a copper surface was clarified. An increase in the humidity results in a decrease in the thickness, in contrast, a decrease in the humidity increases the thickness linearly. Changes in the oxide film thickness based on the level of humidity were measured by ellipsometry. Surface state changes influenced by humidification were analyzed topographically using a scanning tunneling microscope. The mechanism of the effect of humidity on the film thickness was discussed on the basis of the deduction of the copper oxide film by H 2 from the adsorbed H 2 O. Moreover, the changes in contact resistance levels for both static and sliding contacts due to humidity were measured, and a dependence on humidity was found.

Effect of pulsed laser parameters on the corrosion limitation for electric connector coatings

Materials used in electrical contact applications are usually constituted of multilayered compounds (e.g.: copper alloy electroplated with a nickel layer and finally by a gold layer). After the electro-deposition, micro-channels and pores within the gold layer allow undesirable corrosion of the underlying protection. In order to modify the gold-coating microstructure, a laser surface treatment was applied. The laser treatment suppressing porosity and smoothing the surface sealed the original open structure as a low roughness allows a good electrical contact. Corrosion tests were carried out in humid synthetic air containing three polluting gases. SEM characterization of cross-sections was performed to estimate the gold melting depth and to observe the modifications of gold structure obtained after laser treatment. The effects of the laser treatment were studied according to different surface parameters (roughness of the substrate and thickness of the gold layer) and different laser parameters (laser wavelength, laser fluence, pulse duration and number of pulses). A thermokinetic model was used to understand the heating and melting mechanism of the multilayered coating to optimize the process in terms of laser wavelength, energy and time of interaction.

A Model of Electrical Contacts with Advanced Degradation

This paper describes a model of the operation of electric contacts. An equivalent circuit is proposed that is valid for the usual application of electrical contacts. The theoretical background of the model is relations derived from quantum mechanical tunnelling, and a new formula is proposed that describes the activity of arcs and discharges. The investigation is based upon commonly accepted principles of contacts operation. The experimental results and the theoretical results from the model are in close agreement. The proposed model appears to adequately describe the operation of electrical contacts, even badly degraded contacts.

Electroless Ni-B plating for electrical contact applications

Electroless Ni-B plating has been tried on steel substrate in an effort to employ low-cost starting materials for electrical contacts or connectors. By selected conditions of heat treatment in a high vacuum environment the plating can acquire Cr-equivalent hardness without the effluents of the hard chromium plating process. The surfaces were characterized under scanning electron microscope and by XRD. The fabricated materials were tested under corrosion conditions by polarization measurements. Semispherical nickel plated steel joints were tested in a computer controlled contact make-break apparatus, under simultaneous application of a mechanical and a low-voltage electrical load for 20,000 cycles. After heat treatment the plating acquires a crystalline structure with very good adhesion to the substrate material. Corrosion decreases and increased hardness is obtained. The surface is also characterized by good electrical properties during aging accelerated tests.

A novel process for fabricating electrical contact SnO2/Ag composites by reciprocating extrusion

Reciprocating extrusion combined with simple powder compaction was used to produce electrical contact SnO2/Ag composites from Ag and SnO2 powders with improved properties. The SnO2 particles were uniformly dispersed in the Ag matrix after 20 passes of extrusion. The hardness of the composites increased with extrusion passes and leveled off after ten passes, whereas the electrical conductivity decreased slowly after five passes. The variation of the coefficient of thermal expansion with temperature revealed that SnO2/Ag interface strength increased with extrusion passes and was greatest after 20 passes. The SnO2/Ag composites with 20 passes showed the best electrical contact properties in which there was no mass loss through mass transfer from cathode to anode during the make-break erosion test at both low and high current conditions. On the other hand, SnO2/Ag composites with one pass and commercial SnO2-In2O3/Ag specimens had an apparent mass loss under the same condition. Electrical contact composites produced by the present method are promising in electrical contact applications in consideration of cost and performance.

Preparation and characterization of electroplated amorphous gold–nickel alloy film for electrical contact applications

A process for electroplating amorphous gold–nickel alloy with the atomic ratio of unity was developed. The plating bath was prepared by adding potassium cyanoaurate(I) into a known plating bath which produces amorphous nickel-tungsten alloy. At a sufficiently high gold concentration, the alloy deposit did not contain any tungsten. The amorphous nature of the Au–Ni alloy produced in the new bath was confirmed by using TEM and THEED. Hardness, resistivity, and contact resistance of this new alloy were determined, and the results are discussed for applications as an electrical contact material.

Study on self-aligned contact oxide etching using C5F8∕O2∕Ar and C5F8∕O2∕Ar∕CH2F2 plasma

A self-aligned contact (SAC) technology is developed for the application of electrical contacts between the local interconnection and the silicon diffusion regions (or plug pad) from a 0.18μm device. Commercial memory devices have capping (or spacers) nitride (Si3N4) for protection of Word Line (or Bit Line) and a thin nitride film layer for the SAC etch to stop on. The main problems with SAC etch processes in ULSI devices of sub-0.1-μm-design rule are low selectivity to nitride and etching-stop due to high aspect ratio of the contact hole. The key for a successful SAC etch process is control of polymer generation. In this study, the SAC oxide etch will be characterized by C5F8∕O2∕Ar and C5F8∕O2∕Ar∕CH2F2 plasmas. As the ratio of O2 increases in the C5F8∕Ar∕O2 mixture, the amount of polymer decreases and the ability of contact etching increases. The effect of the CH2F2 addition to SAC oxide etching in C5F8∕Ar∕O2 is the selectivity of oxide to nitride, especially to that of nitride contact bottom for SAC etch stopping, increases. The selectivity of oxide to nitride increases with increasing plasma source power or with decreasing bias power. SAC patterned wafers were characterized using top-down critical dimension scanning electron microscopy (CD-SEM) and transmission electron microscopy (TEM). To analyze the effect of the addition of CH2F2 gas to C5F8∕O2∕Ar plasmas, we investigated the chemical species in the gas phase with optical emission spectroscopy (OES). The components and thickness of the fluorocarbon polymer on the contact surface, bottom and sidewall were investigated with transmission electron microscopy, and x-ray photoelectron spectroscopy.

Thermal properties characterization of conductive thin films and surfaces by pulsed lasers

An experimental system for investigation of thermal properties of thin films and material surfaces especially with high thermal conductivity was developed. This system is based on photothermal method using pulsed laser with nanosecond pulse duration in the UV range, and a high speed IR photodetector. Calibration of the experimental system, enabling absolute temperature measurement, is described. The system was calibrated for Cu, Al, Ti, Ni samples and Au/Ni layers on bronze substrates, where the mean sensibility of the detector varied from 31 μV/K for Cu and Au to 116 μV/K for Ti. First are shown measured thermal responses and deduced effusivities for pure metals in a ‘bulk’ form, which were investigated to validate the experimental accuracy of the system. Second, for a specific application of electrical contacts in microelectronics multilayer Au/Ni coatings on bronze substrates were studied. Thermal effusivity values plotted versus time after laser pulse enable estimation of the effusivity variation from the surface coating to the substrate. The effusivity of surface Au/Ni coatings with thickness 0.8/2 μm was estimated to 25 800 W s 1/2 m −2 K −1 and with thickness 0.2/2 μm was estimated to 21 400 W s 1/2 m −2 K −1.