Contact resistance between metals

In 1987, Swartz [73] measured the thermal boundary resistance between metal films and the dielectric substrates onto which the films were deposited, in the range 0.6-200 K. A typical example is the measurement of the thermal contact resistance between indium and sapphire [72]. To minimize the dependence on surface irregularities, indium was vacuum deposited onto the sapphire rods the two surfaces were then pressed together and annealed. Analogous measurements have been carried out also with lead and aluminium. In all these cases, it has been clear that the contact resistance was strongly dependent on the sample preparation. In particular, obtained data suggest that the contact between the two materials was not complete. 

As we said before, the contact resistance between normal metals should theoretically vary as T-1. Experimental results show dependence as T l [88], but also with larger exponents. For example, in the contact between Au/Au and Cu/Cu the contact resistance varies as T 13 [89], It is to be remarked that the Au/Au contact shows a thermal resistance 20 times smaller than that of the Cu/Cu contact (probably because of the presence of copper oxides). 

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

Once the plate starts to corrode, many problems appear to affect performance and durability, even serious failure, of fhe fuel cells. For example, fhe interface contact resistance between the corroded metal plates and GDL will increase to reduce the power output. The corrosion products (mainly various cations) will contaminate the catalyst and membrane and affect eir normal functions because the polymer membrane essentially is a strong cation exchanger and the catalyst is susceptible to the ion impurity. Hence, adding a corrosion-resistant coating to the metal plate will almost inevitably assure the performance and long-term durability of a sfack. 

The assumption that the potential is uniform in the metal of the bed but suffers an IR drop in the solution may be insufficient. Contact resistances between the metal particles set up a significant resistance and hence IR drop in the metal of the bed itself (Kim, 1997). 

The main problem in accurate measurement of resistivity is one of contact resistance between the measurement electrodes and the specimen. This is clearly the case for samples with low resistivity, but can also be a problem for more resistive samples if either the contact resistance is high or the contact is non-ohmic. Contact resistance may be reduced by painting electrodes directly on to the surface of the specimen instead of relying on pressure contact with metal plates or foils. Suitable paints are silver dispersions or Aquadag (an... 

Metal-to-junction contact resistance between CMP and resist etch back (REB) planarized wafers with either silicon or TiSij contacts. (From Ref. (25).)... 

The thermal contact resistance between different components of building structures ranges between 0.01 and 0.1 m °C/W, which is negligible in most cases. However, it may be significant for metal building components such as steel framing members. 

Develop surface treatment processing to minimize the contact resistance between the sensing layer and the underlying metal contact... 

Holm (1967) identifies the contact resistance between particles of clean metal to be the result of current constriction at the point of contact. This geometric constriction together with the volume and surface resistivities integrated over the remaining volume and surface of a particle constitute the total resistance measured between two contacts located at the poles of the particle. In addition, if a thin film exists between the particle contacts, the tunnel effect provides a current independent of the film resistivity. 

Contact Resistance - The resistance between metallic contacts and the semiconductor. 

Dhiman and Chandra [20] developed an analytical model to predict the substrate temperature at which splashing would occur by using a one-dimensional model for solidification of a molten metal droplet in contact with a semi-infinite substrate. They assumed that splashing occurred if the thickness of the solid layer reached that of the splat by the time the droplet had finished spreading. The thermal contact resistance between the droplet and surface was found to play a critical role in... 

An alternative approach is to place Au or Pt electrodes directly in contact with the membrane and measure the resistance using the cell presented in Fig. 4.3.25B. This method may include the contact resistance between the metal and the membrane, and hence mercury electrodes are preferred. 

The contact resistance between two pieces of metal depends on the quality of the surfaces and the pressure that is used to force them together. Increasing the pressure flattens out more of the microscopic irregularities on the surfaces and increases the true contact area. Beyond 20 MPa, gains are small. Beyond 40 MPa, they are inconsequential, and too much pressure on the joint can exceed the elastic limit of the material. Under high pressure and at service temperature, the metal will flow slightly. When it cools, the contact pressure wiU be greatly reduced and the resistance of the joint will increase. Bolted joints are particularly susceptible to this when the bolts and the bars have diffetent thermal coefficients of expansion. From this standpoint, copper alloy bolts are superior to steel. 

Contact resistance between an ICA (generally a Ag-flake-filled epoxy) and nonnoble metal finished components increases dramatically during an elevated temperature and humidity aging, especially at 85°C/85% RH. The National Center of Manufacturing and Science (NCMS) defined the stability criterion for solder replacement conductive adhesives as a contact resistance shift of less than 20% after aging at 85°C/85% RH conditions for 500 h (67). 

The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this distance is increased. Earth electrode resistance consists of the sum of the resistance of the metal electrode (negligible) plus the contact resistance between the electrode and the soil (negligible) plus the soil resistance itself Thus, for all practical purposes, earth electrode resistance equals the soil resistance. The soil resistance is nonhnear, with most of the earth resistance contained within several feet of the electrode. Furthermore, current flows only through the electrolyte portion of the soil, not the soil itself Thus, soil resistance varies as the electrolyte content (moisture and salts) of the soil varies. Without electrolyte, soil resistance would be infinite. 

Immediately before and after the exposure, the contact resistance between the metals in each galvanic couple is checked with an ofammeter. Following exposure, initial cleaning of marine growth is done with a plastic scraper and nylon brush, then transferred to the laboratory for final cleaning. 

A variant of the enhanced reaction zone concept is to utilize as catalyst support various porous three-dimensional electrodes with thickness between 200 to 2,000 pm. Thus, the electric contact resistance between the individual layers is eliminated. The three-dimensional matrix (such as various graphite felts, reticulated vitreous carbon, metal mesh, felt, and foam) supporting uniformly dispersed electrocatalysts (nanoparticles or thin mesoporous coating) could assure an extended reaction zone for fuel (methanol, ethanol, and formie aeid) electrooxidation, providing an ionic conductor network is established to link the catalytically active sites and the proton exchange membrane. The patent by Wilkinson et al. also suggests such electrode configurations (e.g., carbon foam, expended metal and reticulated metal) but experimental results were not provided [303]. 

In the present study, a knee joint simulator is used to simulate the tibial axis load and flexion-extension motion under walking condition. The lubrication condition or fluid film formation in knee prosthesis models is evaluated by measuring the electric contact resistance between the femoral metallic component and tibial conductive polymeric or elastomeric component under the constant applied voltage in a knee joint simulator. 

Measurement Technique. Rectangular stripes of silver paint are applied to the surface of each sample at regularly spaced intervals to form electrical contacts. Wires are attached to the samples by spring-loaded metal clips placed on the silver-paint electrodes. A comparison of two-point and four-point measurements showed that contact resistance between the sample and the silver-paint electrodes can be significant, especially for composites close to the percolation threshold. All measurements reported here are made with a four-point technique [72] in the linear (ohmic) range of the resistance versus voltage characteristic, and shpw no significant time dependence. 

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